What is the answer to a multiplication problem called? Who coined the term?

Product (mathematics) . The Greek for it is the participle γενόμενον, “what has become, what has come into existence”, which I would assume was calqued into Latin as “what is produced”. The LSJ dictionary lists the participle “what has become” for product as being used in Euclid ; but the verb “becomes” for “adds up to…” is already used in Plato.

LSJ reports that δύναμαι “to be capable” was used for “is the result of multiplying two numbers” in Pappus of Alexandria , 600 years after Euclid; so “product” was not the only possible way it could have been named.