CID is an abbreviation for Cubic Inches of Displacement. This unit of measurement is used in reference to internal combustion engines, and measures the total displacement of their cylinders.

Thus, a 5.0L V-8 would be approximately 302 CID. Typically, old V-8 engines are the only ones still measured in this way, since car guys get off on cubic inches. Everything else in the entire auto industry has moved to the metric system.