Meaning of least upper bound

least' up'per bound'

Pronunciation: [key]
— Math. Math.
  1. an upper bound that is less than or equal to all the upper bounds of a particular set. 3 is the least upper bound of the set consisting of 1, 2, 3. Abbr.: lub&hasp; Also called
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.