Tupac Shakur, aka 2Pac, defined NIGGA as an acronym: "Never Ignorant and Getting Goals Accomplished".

Don't look confused, the truth is so plain to see
Cause I'm the nigga that you sell-outs are ashamed to be
In every Jeep and every car, brothers stomp this
I'm Never Ignorant, Getting Goals Accomplished

-- 2Pac, "Violent", 2Pacalypse Now

That's why, when my mother (who is, like me, white) got a letter from one of the teens she works with, and she saw that it said "You are a true NIGGA", she broke down and cried. It was one of the most touching compliments she had ever received.