(n) The front-running leader(s) of an industry; not the imitation spin-offs. Reserved for the select few whom others try to imitate albeit never successfully. If you are not mainline you are hopeless and should probably just quit what you do, now.
(adj) Describing an action that would only be done by someone/something that is mainline.
Guy 1: Why does that guy drive such a shitty old car if he's a lawyer?
Guy 2: He must not be mainline. If he were, he would drive a BMW 5 Series.
Guy 1: Lebron James is overrated.
Guy 2: Seriously?! He is the most mainline basketball player of this generation.