- (chemistry) Initialism of Uridine Diphosphate.
- (computing) Initialism of User Datagram Protocol (jocular: "unreliable datagram protocol," pertaining to the lossy protocol's inherent bias to drop packets under heavy load.)
- (Internet) Initialism of Usenet Death Penalty.
- (Ireland, politics) Initialism of Ulster Democratic Party.
- (military, US) Initialism of Unit Deployment Program.