known quantity

noun

Mathematics. a quantity whose value is given: in algebra, frequently represented by a letter from the first part of the alphabet, as a, b, or c.
any factor, circumstance, etc., that is already accepted or familiar: Her honesty is a known quantity.

Origin of known quantity

First recorded in 1975–80

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018