sum / səm/ • n. 1. a particular amount of money: they could not afford such a sum.2. (the sum of) the total amount resulting from the addition of two or more numbers, amounts, or items: the sum of two prime numbers. ∎ the total amount of something that exists: the sum of his own knowledge.3. an arithmetical problem, esp. at an elementary level.• v. (summed, sum·ming) [tr.] technical find the sum of (two or more amounts): if we sum these equations we obtain x. ∎ [intr.] (sum to) (of two or more amounts) add up to a specified total: these additional probabilities must sum to 1.PHRASES: in sum to sum up; in summary: this interpretation does little, in sum, to add to our understanding.PHRASAL VERBS: sum up give a brief summary of something: Gerard will open the debate and I will sum up. ∎ Law (of a judge) review the evidence at the end of a case, and direct the jury regarding points of law.sum someone/something up express a concise idea of the nature or character of a person or thing: selfish—that summed her up.