—n. 1. the aggregate of two or more numbers, magnitudes, quantities, or particulars as determined by or as if by the mathematical process of addition: The sum of 6 and 8 is 14. 2. a particular aggregate or total, esp. with reference to money: The expenses came to an enormous sum. 3. an indefinite amount or quantity, esp. of money: to lend small sums. 4. a series of numbers or quantities to be added up.
5. an arithmetical problem to be solved, or such a problem worked out and having the various steps shown.
6. the full amount, or the whole.
7. the substance or gist of a matter, comprehensively or broadly viewed or expressed: the sum of his opinions. 8. concise or brief form: in sum. 9. Math. a. the limit of the sequence of partial sums of a given infinite series.
b. union (def. 10a).
10. a summary.

—v.t. 1. to combine into an aggregate or total (often fol. by up).
2. to ascertain the sum of, as by addition.
3. to bring into or contain in a small compass (often fol. by up).

—v.i. 1. to amount (usually fol. by to or into): Their expenses summed into the thousands. 2. sum up, a. to reckon: We summed up our assets and liabilities. b. to bring into or contain in a brief and comprehensive statement; summarize: to sum up the case for the prosecution. c. to form a quick estimate of: I summed him up in a minute.