In a clause that does not have presentational information structure, the focused expression is the expression which could be the answer to a preceding constituent question.
Note that the noun "focus" does not have a stable meaning in linguistics. On the one hand, it may refer collectively to focus constructions, as in Krifka's (2008) well-known statement: "Focus indicates the presence of alternatives that are relevant for the interpretation of linguistic expressions." Here "focus" refers to constructions that "indicate" something, i.e. to kinds of forms or expressions. On the other hand, linguists often say that a construction "indicates focus", i.e. an information-structural meaning.
focus (INF) = in identificational information packaging, the information which is identified as the “filler” for the open part of the presupposed open proposition. Example: in It was Jack who stole my cookies!, the presupposed open proposition is ‘X stole my cookies,’ and the filler of X is Jack – i.e. X = Jack. The focus may be any part of the information in the clause, not just an argument. The term ‘focus construction’ is sometimes used as a synonym for ‘identificational construction,’ but we avoid that usage here. (Section 11.4.1)