[toronto-lisp] Definition of "System"

Here comes my first stranger-in-a-stange-land question. I am busy learning to use asdf. The asdf manual refers to defining "systems." It is not clear to me whether the term "system" has a formal definition in lisp, or rather is used loosely. I see that the CL ansi spec has a chapter (ch. 24) on System Construction, but this does not elucidate much (for me). For asdf purposes, does "system" mean something specific, or does it just mean a bunch of lisp bits that make up a useful chunk of software?