A means of depicting objects in space in a way that closely approximates the real appearance of the world. There are 2 types of perspective, linear perspective first described in 1435 by Alberti. Brunelleschi is credited as being a co-inventor. Aerial perspective is the other type. Leonardo left his notes on his discoveries with this in the years after Alberti.
In linear perspective all horizontal lines converge to a vanishing point on the theoretical horizon (which is equivalent to the artists eye level). There are normally 2 vanishing points.
In aerial perspective colors get closer to the mid tone the further away they are, lines get softer and contrast contracts toward the middle tones. Conversely as objects get closer their lines become sharper, colors intensify, and the contrast range increases.