Definition of Estimation?

[The common definition of estimate is] "the most optimistic prediction that has a non-zero probability of coming true." … Accepting this definition leads irrevocably toward a method called what's-the-earliest-date-by-which-you-can't-prove-you-won't-be-finished estimating
—Tom DeMarco
—Software Estimation: Demystifying the Black Art: The Black Art Demystified, Chapter 3, Opening quip