Carcinisation (American English: carcinization) is a form of convergent evolution in which non-crab crustaceans evolve a crab-like body plan. The term was introduced into evolutionary biology by L. A. Borradaile, who described it as “the many attempts of Nature to evolve a crab”.
Shrimp vs prawn is colloquial, not a scientific classification, so it varies from place to place and person to person, and depending on the specific animal we’re talking about, but in general