In automotive terms, a Strut is a shock absorber (or damper) with a fixed mount at the non-shaft end rather than a pivoting link. In all other ways, it is identical to an ordinary shock absorber.
Struts are most commonly associated with the MacPherson strut suspension, a space, weight, and cost-saving suspension commonly found in the front of lightweight vehicles. It is typically used only for front suspension, but some cars have a MacPherson suspension, and thus struts, at all four corners.
Struts typically have two parts; The body, which is simply a cylinder with the lower strut mount attached to it; and the cartridge, a replaceable shock absorber. This minimizes the cost of replacement. Not all struts are designed in this fashion.