Is it standard to italicize more than one word in a definition? When writing mathematical definitions, people often italicize the word being defined. For example, > A set is called _closed_ if $\ldots$ Is it standard to italicize multiple words in a single definition? For example, > A _solution_ for a set is $\dots$ A set is called _solveable_ if a solution exists for that set.

It is standard, because it is helpful, to italicize the word being defined in a definition. In the present case, two words are being defined; so two italicizations are appropriate. The author chose, though, to put the two definitions under one heading since they are so closely related.