Atmospheric (aerial) perspective
Like linear perspective, this is a technique for creating an illusion of space on a flat surface.  The illusion is created in two ways:
1) by making forms in the distance less distinct than forms in the foreground
2) by making forms in the background less intense in color than forms in the foreground.

A French military term for the vanguard or advanced guard, it was appropriated for artistic usage in early 19th-century France to describe art that was at the forefront of artistic development. The concept originated in socialist political theory and its first major artistic exponent was Gustave Courbet in his Realist paintings of the 1850s. Today avant-garde is almost synonymous with modern. (The Concise Oxford Dictionary of Art Terms, Michael Clarke, Deborah Clarke. © 2012 Oxford University Press. Available at Oxford Art Online. )

One-dimensional: refers to a point that is infinitely small.
Two-dimensional: refers to a plane that is infinitely thin, flat.
Three-dimensional: refers to the extension of a two-dimensional plane along a third axis perpendicular to its length and width.

An etching is made by covering a copper or zinc plate with wax or resin then removing lines of wax by drawing into it with a sharp implement thus exposing the metal. The plate is then placed in an acid bath. The acid bites into the metal, eating into the plate where it is exposed, the rest of the plate is protected by the wax. Next the acid is washed from the plate and the plate is heated so the wax softens and can be wiped away. The plate now has recessed lines etched by the acid where the artist had drawn into the wax. The plate is inked and the surface wiped clean so that the ink only remains in the recessed areas. Paper is pressed against the plate in a press. The ink adheres to the paper and is drawn out by it. This print, which mirrors the image on the plate, is an etching.

If we think of the flat surface that the picture is on as the picture plane, then we can see foreshortening is when something appears to be (or has the illusion of being) perpendicular (or close to perpendicular) to the picture plane. Another way of saying this is that with foreshortening, something appears to be coming out of the space of the picture towards us, or going back into the space of the picture away from us. Foreshortening creates an illusion of space on the flat surface of the picture, and this makes the image look more real. On the other hand, when forms are parallel to the picture plane, the flatness of the picture plane is reinforced.

Andrea Mantegna, Dead Christ, tempera on canvas, c. 1480 - 1500 (Pinacoteca di Brera, Milan)

Andrea Mantegna, Dead Christ, tempera on canvas, c. 1480 – 1500 (Pinacoteca di Brera, Milan)

When referring to sculpture, the term “free-standing” refers to a sculpture that is not attached to architecture or inside a niche. You can walk around a free-standing sculpture and look at it from all angles. Medieval sculpture was almost always attached to architecture, that is, it adorned the interior and exteriors of churches, whereas we often see  free-standing sculpture in classical antiquity and again, in the Renaissance.

Hieratic scale (or hierarchy of scale)
Representing the sizes of figures according to their importance, rather than how they would objectively appear in reality.  Hieratic scale is often seen in the art of various ancient civilizations, as well as during Europe’s Middle Ages.

Linear perspective 
Linear Perspective is a system for creating an illusion of three dimensional space on a flat, two dimensional surface. It involves creating a horizontal line (called the horizon line), and a point on the horizon line (called the vanishing point), and diagonal lines which appear to recede in space (called orthogonals) which all meet at the vanishing point.  Artists use linear perspective to create an illusion of space from a single, fixed viewpoint. The multiple viewpoints, rising ground line, and ambiguous space of the middle ages were replaced with a rational, measured spatial illusion.  Brunelleschi discovered linear perspective around 1420 in Florence, and it was described by the architect and Humanist Leon Battista Alberti in his book On Painting, which was published in 1435.  Masaccio’s fresco The Holy Trinity (c. 1425) is the earliest surviving example of one-point linear perspective. For more on linear perspective watch this video).

A word used to describe works of art that look realistic, that imitate the natural world as closely as possible.

The mathematical relationship of the parts in any composition to each other and to the whole. More specifically, it refers to the mathematical and geometric relationships of the parts of the human body and the ratio of each part or unit of parts to the whole mass and form. The proportions of the human body have been debated throughout the history of art, the most famous early treatise being Vitruvius’s De Architectura written in the 1st century B.C.E. Major Renaissance studies of the subject included those by Leonardo da Vinci and Albrecht Dürer. (The Concise Oxford Dictionary of Art Terms, Michael Clarke, Deborah Clarke. © 2012 Oxford University Press. Available at Oxford Art Online)

A term devised by Marcel Duchamp to describe pre-existing, mass-produced objects, selected at random, which were then accorded the status of works of art. His first ready-made was a bicycle wheel mounted on a stool (1913). Ready-mades differed from objets trouvés (found objects such as stones, shells, etc.) as the latter were chosen for their aesthetic beauty. Perhaps the most notorious ready-made was Fountain (1917), consisting of a urinal which Duchamp signed ‘R. Mutt’. The ready-made was one of Dada’s most enduring legacies to modern art and was adopted by both Nouveau Réalisme and Pop art. (The Concise Oxford Dictionary of Art Terms, Michael Clarke, Deborah Clarke. © 2012 Oxford University Press. Available at Oxford Art Online)

A term used to describe the light, elegant, and sensuous style in the visual arts which originated in France at the beginning of the 18th century, reached its apogee in the 1730s, and was eventually supplanted by the stern, moralizing qualities of Neoclassicism in the 1760s. Like so many terms of stylistic or period definition it was originally pejorative: it was said to have been coined in the 1790s by one of the pupils of the great Neoclassical painter Jacques-Louis David to refer disparagingly to the art produced during the reign of Louis XV. The word Rococo was apparently a combination of rocaille and barocco (Baroque). It was used formally as an art-historical term from the middle of the 19th century. It is now generally accepted, though its exact definition has been much debated. Purists might argue that Rococo, with its love of shell-like curves and S- and C-curves, was essentially a style of decoration and should be applied to art forms such as boiserie, metalwork, furniture, and porcelain. Today its wider use is generally countenanced, however, and it is also applied to painting, sculpture, and architecture. Watteau is widely considered to have been the first great Rococo painter and Boucher and Fragonard the masters of its mature style. In sculpture Falconet has often been thought the pre-eminent Rococo practitioner and many of his designs were reproduced in porcelain. In architecture its influence spread rapidly abroad, particularly to southern Germany and Austria where it is evident in churches such as Vierzehnheiligen and Die Wies. However, certain architectural historians would argue that this Germanic Rococo was in fact a manifestation of late Baroque rather than a new stylistic development. (The Concise Oxford Dictionary of Art Terms, Michael Clarke, Deborah Clarke. © 2012 Oxford University Press. Available at Oxford Art Online)

An aesthetic concept which entered mainstream European thought in the 18th century. As a category it was distinct from, though often discussed in conjunction with, the Beautiful and the Picturesque, both in relation to aesthetics and, in Britain, to landscape gardening. It originally derived from rhetoric and poetry and gained wider currency after the translation (1674) into French of the Greek treatise On the Sublime, attributed to Longinus (1st century ad). The major work in English on the subject was Edmund Burke’s Philosophical Enquiry into the Origin of our Ideas of the Sublime and the Beautiful (1757) in which the Sublime was differentiated from the Beautiful by virtue of its ability to evoke more intense emotions through vastness, a quality that inspires awe. Travellers came to visit wild and rugged mountainous regions such as the Alps, Snowdonia, and the Lake District in search of the emotional thrills provided by the Sublime, and artists such as J. M. W. Turner responded to the demand for such imagery. Subjects from Homer, Milton, and Ossian were also considered suitable subject-matter in this context. Whereas Burke had considered the Sublime as an external force inherent in the properties of certain objects and nature, the German philosopher Immanuel Kant, most famously in his Critique of Judgement (1790), internalized it and focused on the individual’s response, his contention being that the Sublime came from within the human psyche. A number of theorists and artists of the later 20th century have shown a revived interest in the Sublime. The Concise Oxford Dictionary of Art Terms, Michael Clarke, Deborah Clarke. © 2012 Oxford University Press. Available at Oxford Art Online