not changing or varying; uniform; regular; invariable

All conditions during the three experiments were constant.

continuing without pause or letup; unceasing

constant noise.

regularly recurrent; continual; persistent

He found it impossible to work with constant interruption.

faithful; unswerving in love, devotion, etc.

a constant lover.

steadfast; firm in mind or purpose; resolute.

Obsolete . certain; confident.

something that does not or cannot change or vary.

Physics . a number expressing a property, quantity, or relation that remains unchanged under specified conditions.

Mathematics . a quantity assumed to be unchanged throughout a given discussion.

adjective

changeable , fickle , fluctuating , inconstant , irregular , unstable , unsteady , varying , wavering , concluding , ending , interrupted , stopping , terminable , terminating , disloyal , flagging , undecided , undetermined , untrue , untrustworthy

adjective

connected , consistent , continual , equable , even , firm , habitual , homogeneous , immutable , invariable , like the rock of gibralter , monochrome , monophonic , monotonous , nonstop , of a piece , permanent , perpetual , regular , regularized , solid as rock , stabile , stable , standardized , steadfast , steady , together , unalterable , unbroken , unchanging , unflappable , unfluctuating , uniform , uninterrupted , unvarying , abiding , ceaseless , chronic , continuous , endless , enduring , eternal , everlasting , incessant , interminable , lasting , persistent , persisting , relentless , sustained , unending , unrelenting , unremitting , allegiant , attached , dependable , devoted , dogged , faithful , fast , persevering , resolute , staunch , tried-and-true , true , trustworthy , trusty , unfailing , unflagging , unshaken , unwavering , around-the-clock , ongoing , round-the-clock , timeless , unceasing , changeless , same , invariant , determined , stiff , tough , unbending , uncompromising , unflinching , unyielding , liege , loyal , certain , confident , continent , durable , fixed , forever , inveterate , perennial , solid , stalwart , stanch , undeviating , unfading , unfaltering , unregenerate , unswerving , untiring