Rigid
Hard
Synonyms: Stiff, firm, non-flexible, unyielding, taut, impliable, hard, inflexible, unbending, inelastic, tight, and unmalleable.
Inflexible
Synonyms: Fixed, firm, unalterable, immutable, invariable, cast-iron, set, inflexible, unchangeable, unvarying, and hard and fast.
Stringent
Synonyms: Strict, stern, rigorous, uncompromising, determined, unshakeable, intransigent, unwavering, obdurate, adamant, severe, stringent, inflexible, resolute, immovable, unrelenting, unyielding, unswerving, and unadaptable.
Example Sentences
The structure was surprisingly rigid and strong.
His beliefs are quite rigid and inflexible.
The plastic became rigid when cooled.
She followed a rigid daily schedule.