Maingaining is eating at your maintenance calories or near maintenance, so as to minimize fat gain while still gaining muscle. It is a consistent and steady health focused method of gaining muscle, popularized largely by Greg Doucette.
Kyle: "Yo Derek, you see the size of Amanda's majongalonghallows?"
Mark: "Yeah her majongalonghallows are f*cking massive, bro."
Derek: "Word, I'll have tocheck out Amanda's jugs!"