first language

From first +‎ language.

first language (plural first languages)

1. The first language one is taught to speak; one's native language.

   She was born and raised in France, although her first language is Portuguese because her parents are from Brazil.

   Synonyms: native language, mother tongue, mother language

   • 2018, Clarence Green, James Lambert, “Advancing disciplinary literacy through English for academic purposes: Discipline-specific wordlists, collocations and word families for eight secondary subjects”, in Journal of English for Academic Purposes, volume 35, →DOI, page 105:

     Vocabulary is a vital component of educational success in both first and second language contexts.

2. The language one feels most comfortable and capable with.
3. Used other than figuratively or idiomatically: see first,‎ language.

• mother tongue
• second language

• “first language”, in Lexico, Dictionary.com; Oxford University Press, 2019–2022.