SenecaPronunciation: /ˈsɛnɪkə/Definition of Seneca noun (plural same or Senecas) 1a member of an American Indian people that was one of the five nations comprising the original Iroquois confederacy. 2 [mass noun] the Iroquoian language of the Seneca, now with few speakers. adjective relating to the Seneca or their language. Origin:via Dutch from AlgonquianSeneca in other Oxford dictionaries Definition of Seneca in the US English dictionary