Any argument carried out with sufficient precision.All of these answers are unsatisfactory.
Using arguments with more than two steps.
Mathematics is what mathematicians do.
Mathematics is the branch of natural philosophy that concerns itself with only making true statements.
Mathematics ought to be considered as a set of precise, symbolic, languages that serves as a lingua franca for the physical sciences.
Mathematics is knowledge obtained by logical proofs.
Saying that mathematics is a language is like saying music or philosophy is a language. Sure they use language to communicate, but so does everyone else.
Using phrases like "sufficient precision" ignores the fact that some arguments are proofs, and some are not. Math demands proofs.
Saying "true statements" comes the closest to describing math, but of course many other fields claim to be finding truth. Only math finds it with logical proofs.