Lewis & Short

ūsurpātor, ōris, m. [usurpo, II. B. 2.],

  1. I. one who uses or takes possession unlawfully, a usurper of a thing (late Lat.): indebitae potestatis, Amm. 26, 7, 12: tanti nominis, Symm. Or. ap. Val. 1, 22.
  2. II. In gen., one who accomplishes: ardui operis, Jul. Sev. Syn. praef. 1.