1 resolute, immovable, steadfast, firm, dependable, unshakeable or unshakable, determined, unswerving, undeviating, persevering, unwearying, unwearied, untiring, indefatigable, tireless, unflagging, unwavering, unfailing, unfaltering, persistent; loyal, true, tried and true, devoted, staunch, trusty, faithful:
He was her constant companion during her troubles.
2 incessant, unceasing, ceaseless, perpetual, persistent, uninterrupted, steady, regular, invariable, unremitting, unvarying, relentless, unrelenting, continuous, continual; unending, endless, never-ending, non-stop, perennial, eternal, everlasting, Literary sempiternal:
The constant pain almost made me cry out. Their constant bickering is getting on my nerves.
3 unchanging, unchanged, invariable, unvarying, fixed, uniform, unalterable, immutable, changeless, persistent:
The numbers might change, but the ratio is constant.