n.,adj. Often capitalized; always pronounced as if capitalized.
1. Self-evidently wonderful to anyone in a position to notice:
"The Trailblazer's 19.2Kbaud PEP mode with on-the-fly Lempel-Ziv compression is a Good Thing for sites relaying netnews."
2. Something that can't possibly have any ill side-effects and may save considerable grief later:
"Removing the self-modifying code from that shared library would be a Good Thing."
3. When said of software tools or libraries, as in "YACC is a Good Thing", specifically connotes that the thing has drastically reduced a programmer's work load.
Oppose Bad Thing.