1. (Or "finite", "isolated") In domain theory, an element d of a cpo D is compact if and only if, for any chain S, a subset of D,
d <= lub S => there exists s in S such that d <= s.
I.e. you always reach d (or better) after a finite number of steps up the chain.
("<=" is written in LaTeX as sqsubseteq).
2. Of a design, describes the valuable property that it can all be apprehended at once in one's head. This generally means the thing created from the design can be used with greater facility and fewer errors than an equivalent tool that is not compact. Compactness does not imply triviality or lack of power; for example, C is compact and Fortran is not, but C is more powerful than Fortran. Designs become non-compact through accreting features and cruft that don't merge cleanly into the overall design scheme (thus, some fans of Classic C maintain that ANSI C is no longer compact).