Confusion exists regarding dampers, gas struts, gas springs, gas shocks, and many other words used on somewhat inter-changed basis. These terms in fact do distinguish different products. The fact that these products often look similar externally lends to the confusion. Some basic parameters that apply to all of these products are:

Length:
From center of mounting to center of mounting point for the product. Typically extended or compressed length specified. (EL in diagram corresponds to extended length)

Stroke:
The amount of travel available in the product. (S in diagram)

Diameter:
The device may have one or more diameters specified depending on the configuration. (Dr in diagram corresponds to rod diameter, and Db correspond to body diameter)