To seize and hold possession of, as of some important or dignified place, office, power, or property, by force or without right; seize, appropriate, or assume illegally or wrongfully: as, to usurp a throne; to usurp the prerogatives of the crown; to usurp power.
To assume, in a wider sense; put on; sometimes, to counterfeit.
To be or act as a usurper; hence, to commit illegal seizure; encroach: with on or upon.