All 9 Uses

ion
in
The Martian
(Auto-generated)

Hermes is powered by ion engines.†
p. 2.7 *

ion = an atom that has acquired an electrical charge by the loss or gain of an electron
I know you get bored when I go into the math, so I'll summarize: The small, constant thrust of Hermes's ion drives is much harder to deal with than the large point-thrusts of presupply probes.†
p. 176.3
Hermes has a constant-thrust ion engine.†
p. 201.5
So long as the reactor and ion engines continued to work, we could bring it back.†
p. 204.1
It knew when to angle the ship so the ion engines would be aimed correctly.†
p. 208.4
It was the nuclear-powered ion engine that took me to Mars†
p. 252.2
"Vogel," Lewis continued, "how far can we deflect in thirty-nine minutes with the ion engines?"†
p. 348.9
Just the slow, steady ion engines.†
p. 349.9
"The ion drive is simply not strong enough," Vogel replied.†
p. 351.1

Definitions:

(1) (ion)
an atom that has acquired an electrical charge by the loss or gain of an electron
(2) (meaning too rare to warrant focus)