A page from the Unix Programmer's Manual, documenting one of Unix's many commands, system calls, library subroutines, device driver interfaces, file formats, games, macro packages, or maintenance utilities. By extension, the term “man page” may be used to refer to documentation of any kind, under any system, though it is most likely to be confined to short on-line references.
As mentioned in Chapter 11, Other Lexicon Conventions, there is a standard syntax for referring to man page entries: the phrase “foo(n)” refers to the page for “foo” in chapter n of the manual, where chapter 1 is user commands, chapter 2 is system calls, etc.
The man page format is beloved, or berated, for having the same sort of pithy utility as the rest of Unix. Man pages tend to be written as very compact, concise descriptions which are complete but not forgiving of the lazy or careless reader. Their stylized format does a good job of summarizing the essentials: invocation syntax, options, basic functionality. While such a concise reference is perfect for the do-one-thing-and-do-it-well tools which are favored by the Unix philosophy, it admittedly breaks down when applied to a command which is itself a major subsystem.