rigid, rigorous, strict, stringent mean extremely severe or stern. rigid implies uncompromising inflexibility.

Rigid means stiff and inflexible. It can be used in both literal and figurative ways.For example, a material like plastic might be described as rigid if it does not bend or bend easily.

rigid adjective (STIFF) not able to be bent: rigid plastic (Definition of rigid from the Cambridge Academic Content Dictionary © Cambridge University Press)

A rigid substance or object is stiff and does not bend, stretch, or twist easily. ...rigid plastic containers. These plates are fairly rigid.

Definition of rigid adjective from the Oxford Advanced Learner's Dictionary. (often disapproving) (of rules, methods, etc.) very strict and difficult to change synonym inflexible. The curriculum …

1. not bending; physically inflexible or stiff: a rigid piece of plastic. 2. unbending; rigorously strict; severe: rigid rules.

The periodical has rigid [= strict, inflexible] guidelines for submissions. He is a rigid disciplinarian. The law was rigidly enforced. The soldiers stood rigidly at attention.

Describe something as rigid if it's really stiff and not very flexible, like a super-strict practice schedule or an unbending, uncomfortable mattress.

Not easily wrought upon or affected; inflexible; hence, harsh; severe; rigorous; rigorously framed or executed: as, a rigid sentence; rigid criticism. Strict in opinion, conduct, discipline, or …

There are 21 meanings listed in OED's entry for the word rigid, one of which is labelled obsolete. See ‘Meaning & use’ for definitions, usage, and quotation evidence.