Meaning of inertial upper stage

iner'tial up'per stage'

Pronunciation: [key]
  1. a U.S. two-stage, solid-propellant rocket used to boost a relatively heavy spacecraft from a low earth orbit into a planetary trajectory or an elliptical transfer orbit. Abbr.: IUS Cf. payload assist module.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.