In a vehicle, a piece of the front suspension connecting the steering knuckle to the rack and pinion or, in a non-power-steering-enabled car, the steering gearbox, the term for which escapes me at this moment. The tie rod has, on either end, swivel joints which are referred to as the inner tie rod end, and the outer tie rod end.

As your tie rod ends start to wear out, your front wheels will start to turn ever so slightly by themselves, requiring minor, frequent steering corrections. While this in and of itself is not dangerous, it will cause advanced tire wear.

The usual maintainance procedure for a tie rod end involves using a grease gun to pack grease into it(the tie rod end) through a special fitting called a grease zurt - which is, in fact, what most mechanic-type establishments refer to as a chassis lube. It's also customary to inspect the seals when performing this procedure.