/seuh stay"neuhr/ , n.
1. a person or thing that sustains.
2. Rocketry.
a. any stage of a multistage rocket or guided missile that sustains flight after the burnout of the booster.
b. the rocket engine or cluster of engines contained in such a stage.
[ 1350-1400; ME sosteynere. See SUSTAIN, -ER 1 ]