pertinacity

noun

Etymology: From Middle French pertinacité, from Old French pertinace (“obstinate, stubborn”).

1. The state or characteristic of being pertinacious.

   “1846, Edgar Allen Poe, The Black Cat With my aversion to this cat, however, its partiality for myself seemed to increase. It followed my footsteps with a pertinacity which it would be difficult to make the reader comprehend.”

   “Again and again, however, and half a dozen other agains, with the inexorable pertinacity of a child intent upon some object important to itself, did he renew his efforts for admittance.”