A sprocket or sprocket-wheel is a profiled wheel with teeth that meshes with a chain. Bicyclists normally call the front sprocket attached to the cranks the "chainwheel" and the rear sprockets either "sprocket" or "cassette."
A sprocket or sprocket-wheel is a profiled wheel with teeth [...] that mesh with a chain, track or other perforated or indented material. The name 'sprocket' applies generally to any wheel upon which radial projections engage a chain passing over it. It is distinguished from a gear in that sprockets are never meshed together directly, and differs from a pulley in that sprockets have teeth and pulleys are smooth. Source: wikipedia.