In general, under old English law, someone over whom his or her master has complete dominion regarding what work shall be performed and how it shall be performed. Typically in genealogy we find servants bound to masters by Indenture (or “indentured servants”) who may be either adults or minors, and may not be learning a skill or trade --- but otherwise are serving under similar conditions to an Apprenticeship. Frequently these indentured servants were young immigrants paying off their debt for transportation to the new world while earning their room, board, and clothing in the process.
See also Immigrant Servants Database.
Source: Black's Law Dictionary, 4th Edition.