n.
Pronunciation: ' ə n-d ə r- ˌ grau ̇ nd
Function: noun
Date: 1594
1 : a subterranean space or channel
2 : an underground city railway system
3 a : a movement or group organized in strict secrecy among citizens especially in an occupied country for maintaining communications, popular solidarity, and concerted resistive action pending liberation b : a clandestine conspiratorial organization set up for revolutionary or other disruptive purposes especially against a civil order c : an unofficial, unsanctioned, or illegal but informal movement or group especially : a usually avant-garde group or movement that functions outside the establishment