A stowaway was an individual who gained entry to a starship without permission.
When Captain Jonathan Archer told Matthew Ryan about the Nausicaan bio-signs T'Pol picked up aboard the ECS Fortunate, Ryan considered a stowaway aboard his freighter but was read by Archer who told him not to tell the truth. (ENT: "Fortunate Son")
In 2270, Commander Demos was considered a stowaway aboard the USS Enterprise. After the accusations about Leonard McCoy were cleared up, James T. Kirk promised to forget about Demos' intrusion. (TAS: "Albatross")
In 2369, Jadzia Dax compared the possible presence of Rao Vantika in Ty Kajada's mind to that of a stowaway. (DS9: "The Passenger")