Nouns

(n)constant, constant quantity, invariablea quantity that does not vary
(n)constanta number representing a quantity assumed to have a fixed value in a specified mathematical context“the velocity of light is a constant”

Adjectives

(a)constantsteadfast in purpose or devotion or affection“a man constant in adherence to his ideals”, “a constant lover”, “constant as the northern star”
(s)changeless, constant, invariant, unvaryingunvarying in nature“maintained a constant temperature”, “principles of unvarying validity”
(s)ceaseless, constant, incessant, never-ending, perpetual, unceasing, unremittinguninterrupted 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”