Bendix is the flywheel drive mechanism mounted on the shaft of the starter motor..it is actually its own assembly and can be repalced w/o replacing the starter..so can the starter solinoid. But for practical purposes, rebuilt starters come with both a new solinoid and Bendix drive as a unit...so most shops just replace the whole unit.
It is on the lower/right side of the motor...You may want to jumper from another battery...sometimes the extra boost will free up a stuck bendix drive.
Short cycling with the key....waiting each time for the starter to come to a complete stop.

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A Dalton