A programming language (actually, a Page Description Language, or PDL) developed by Adobe for printing documents on laser printers (it can also be adapted to produce images on other types of devices).
PostScript is the standard in desktop publishing; it is an object-oriented language that treats images and fonts as collections of objects rather than as bitmaps. Apple helped make PostScript popular by selling printers with built-in PostScript interpreters. Many programs have evolved to produce PostScript programs as their output, making it the lingua franca of printing.
One of the benefits is that it is "resolution independent," meaning it can support the highest resolution of your device (printer) and yet be displayed on a low-resolution screen.