brother, friend, a friend that you're so close with, in fact, that you consider him a brother. peer, or otherwise, any person really. the word "nigga" can be used in a variety of ways, but be warned, if misused, there will be consquences. this word is not to be confused with "nigg**" which is fighting words, anywhere, no matter what.
"Ay 6, wuzz craccin nigga
"Juzz kiccin it fam, wut da play is?" - Ronaldo
"Quicc licc, tryna get this cake rightt, feeme?" - Carlito