mother tongue

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from The Century Dictionary.

• noun One's native language.
• noun A tongue or language to which other languages owe their origin.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The language one first learned; the language one grew up with; one's native language.
• noun The language spoken by one’s ancestors.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun one's native language; the language learned by children and passed from one generation to the next