An easter egg--the kind you find in Mac games, anyway--is any sort of
hidden surprise (hence the name) that occurs when an unlikely set of
conditions are met.  For example, when the system clock is set to
December 25, Maelstrom displays Christmas ornaments in the title screen.
Another example is the very large literal easter egg suspended over a
remote lake in F/A-18.  A list of Macintosh and Newton easter eggs compiled
by Brian Kendig <bskendig@netcom.com> can be found at info-mac at
<ftp://mirrors.aol.com/pub/info-mac/info/mac-newton-easter-egg-list-94.txt>.

Daniel Fanton <ixist@aol.com> maintains a comprehensive web site devoted 
to Apple Easter Eggs at <http://users.aol.com/ixist/easter-eggs.html>, 
and there's a cross-platform Easter Egg Archive page at 
<http://weber.u.washington.edu/~davidnf/eggnorm.html>.

Of course, if the programmer didn't put it in on purpose, it's not an
easter egg, it's a "feature."