Non-Government Organization. In this context a non profit that operates independently from governments to address social or political issues.
Accurate dictionary definition. That "independently" doesn't always play off IRL, with the NGO working the grey area and doing things for a gov agency that said agency would get an IG investigation for.
That really got its legs under Obama. in my AO of the gov, I first saw the political appointees with an NGO background, then they hired their NGO buddies at the SES level, then those guys hired GS-15s, and that's where the NGOs really get entrenched in a deep state kinda way. Within a couple of years, it was quite common for me to run into GS-11s thru 14s whose last gig was Environmental Defense Center or similar, and, "Yeah, GS-15 Bob who I used to work for brought me in."