Real Dictionary

sum
countable noun: A sum is a simple calculation in arithmetic.
countable noun: A sum of money is an amount of money.
singular noun: In mathematics, the sum of two numbers is the number that is obtained when they are added together.
singular noun: The sum of something is all of it.

sum in American English
an amount of money
surface-to-underwater missile
intransitive verb: to amount
noun: the aggregate of two or more numbers, magnitudes, quantities, or particulars as determined by or as if by the mathematical process of addition
transitive verb: to combine into an aggregate or total

sum in British English 1
noun: the result of the addition of numbers, quantities, objects, etc
verb: to add or form a total of (something)

sum in British English 2
noun: the standard monetary unit of Uzbekistan, divided into 100 tiyin

sum- in American English
sub-
prefix