Hey Mr. Phil,
I hope I didn't come across wrong in my earlier reply. I went back and read it just now. I certainly didn't mean to.
Anyway, I would define intonation as it relates to a bass or a guitar as being "the accuracy in which a guitar or bass guitar can produce a fretted note at each interval. Setting the intonation is the act of adjusting the length of the strings (by moving the bridge saddles) to compensate for the stretching of a string due to pushing it down to the fret board to produce a note. To adjust the intonation of your guitar or bass, you move the bridge saddles toward or away from the fret board until the 12th fret octave and its harmonic are equal and the same open-string note is exactly one octave below those. Accurate intonation is critical to sound quality".
(disclaimer - the "definition" as posted here was unabashedly plagarized from [Invalid or Expired Link Removed])