Cid/ˌsiː aɪ ˈdiː/ Definition 1 of 3Pro Tip 1/3UK Specific Primarily used in British English, less recognized as a term for detective in American English. In the UK, the CID handles serious crimes. Pro Tip 2/3Professional Role Represents a professional within the police, not used for private detectives. The CID officer presented her badge. Pro Tip 3/3Serious Cases Often associated with more serious or complex criminal investigations. A CID investigation usually means a big case.