How do you define PostScript?
Charles Geschke: PostScript is a programming language whose primary purpose is to provide a high-level description of the content of (printed) pages in a device-independent representation.
John Warnock: PostScript is an interpretive programming language that emulates a simple, stack-oriented virtual machine. In addition to the normal operators found in most programming languages, PostScript has a very rich set of image, graphic, and font rendering operators. With PostScript, an application can emit PostScript commands in a resolution-independent manner that will define the appearance of a printed page (or display).
I think PostScript was successful because it is so flexible and has a well-defined underlying imaging model. Other printer protocols of the time attempted to statically define pages with data structures. These protocols invariably would fail at describing some intuitively simple pages.
What made you create the language PostScript and not a data format? ...