Definition of "pertinacity"

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.

1846, E.A.Poe, The Black Cat

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.

1851 April 9, Nathaniel Hawthorne, The House of the Seven Gables, a Romance, Boston, Mass.: Ticknor, Reed, and Fields