Is "rigid" an adjective?

Yes, rigid is an adjective, it is word that describes noun, indicating stiff, inflexible, not bending, or strict, used to describe physical objects, rules, or people, similar to describing storage as rigid case. As adjective, rigid modifies noun, for example rigid box meaning box that does not bend, rigid rules meaning strict rules without flexibility, rigid person meaning inflexible person resistant to change, rigid foam meaning hard foam that holds shape, rigid schedule meaning fixed schedule with no changes. Rigid comes from Latin rigidus meaning stiff, and functions as descriptive adjective, with comparative more rigid and superlative most rigid, and adverb form rigidly meaning in rigid manner. For example, sentence The rigid structure cannot bend uses rigid as adjective describing structure. So rigid is adjective describing quality of stiffness, inflexibility, or strictness, whether physical like rigid material that does not deform, or behavioral like rigid thinking that does not adapt, so yes rigid is adjective, not noun or verb, used to describe nouns with meaning of stiff, inflexible, and unyielding.