WordNet sense 1 (a quantity that does not vary):

constant quantity

WordNet sense 2 (persistent in occurrence and unvarying in nature):

changeless, unvarying, invariant

WordNet sense 3 (not liable to or capable of change):

invariable

WordNet sense 4 (uninterrupted in time and indefinitely long continuing):

ceaseless, incessant, perpetual, unceasing, unremitting, never-ending

WordNet sense 5 (continuing in time or space without interruption):

continuous, uninterrupted

WordNet sense 6 (continually recurring or continuing without interruption):

continuant

WordNet sense 7 (seemingly without interruption; chiefly restricted to what recurs regularly or frequently in a prolonged and closely spaced series):

continual

WordNet sense 8 (firm and dependable especially in loyalty):

staunch, steadfast, unswerving