I had always thought that, where both are incorporated entities, firms tend to be professions: law, medicine, accountancy. I see, however, that it also has another definition: A commercial partnership of two or more persons, especially when unincorporated . Well, that gibes somewhat with my image, since law firms and medical firms tend to be partnerships.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.