Meaning of known quantity

known' quan'tity

Pronunciation: [key]
  1. 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.
  2. any factor, circumstance, etc., that is already accepted or familiar: Her honesty is a known quantity.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.