Princeton's WordNet

1. ceaseless, constant, incessant, never-ending, perpetual, unceasing, unremitting(adj)

   uninterrupted in time and indefinitely long continuing

   "the ceaseless thunder of surf"; "in constant pain"; "night and day we live with the incessant noise of the city"; "the never-ending search for happiness"; "the perpetual struggle to maintain standards in a democracy"; "man's unceasing warfare with drought and isolation"; "unremitting demands of hunger"