Definition of "mother tongue" []

• One's native language. (noun)
• A parent language. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "Before us squats her super grass a bearded Afro-Arab merchant seaman who for the price of what I'm earning in a year will dish the dirt on a ring of corrupt customs officers and policemen operating in Liverpool's dock land He speaks only meagre English, his mother tongue being a classical Tanzanian-flavoured Swahili."
• "Succeeding governments in both Italy and Spain have turned a blind eye, according to temperament or expedience, and to this day San Juan remains — on Italian territory in Italian seas — Spanish in thought and flavour; still using in highly bastardized form its founder’s mother tongue and strictly upholding and maintaining his deplorable standards."
• "In the ninth century, through the efforts of Sts. Cyril and Methodius and their disciples, the Moravians and the Bulgarians were converted to Christianity, and as the missionaries were Byzantines they introduced their own rite, but translated the Liturgy into Slav, the mother tongue of those nations."