1. A combat situation in which an attacker, normally equipped with a superior weapons system, is presented with a large number of highly desireable, poorly defended and high-value targets all at once, such a situation might be an attack helecopter with night vision capability finding a complete enemy brigade or divisional HQ convoy stalled at night on a narrow road with no place to go and no air defence.
2. A workplace, bar, party or other place of entertainment where a new arrival finds a large number of attractive, unattached members of the opposite sex, and is spoilt for choice
"God, I was in the pub last night and this hen party wandered in.... a dozen pissed girls looking for fun... that's what I call a target rich environment !"
A sarcastic term for being surrounded and/or severely outnumbered by enemies.
Those "Black Hawk Down" guys sure were in a target rich environment.