A True Alpha is a werewolf who rises to Alpha status solely on the strength and purity of their character, sheer force of will. True Alphas are rare, only appearing once in a 100 years.
In Smoke and Mirrors, Peter relays the a True Alpha's status cannot be stolen, usurped were they to be slain as in the manner of a regular Alpha. Furthermore, in Parasomnia, Dr. Deaton says a True Alpha's power cannot be absorbed, siphoned by any being - except for a Beta of said True Alpha's making.
In Season 3, Scott McCall's eyes, during situations of high duress but immense determination, briefly turn Alpha red. In Currents, Dr. Deaton confirms Scott has the potential to be a True Alpha. In Lunar Ellipse, Scott attains the rank of Alpha by determinedly pushing through a mountain ash barrier, successfully breaking it, fully awakening his True Alpha status.