Definition
The word for both "language" (a system of communication) and "tongue" (the muscle in the mouth). Context usually makes the meaning clear.
Example
Она учит новый язык каждый год.
Show translation
She learns a new language every year.
Etymology
From Proto-Slavic *językъ, meaning both "tongue" and "language" — a pattern shared across Slavic languages (Polish język, Czech jazyk). The anatomical and linguistic senses were never separated, reflecting the ancient view that speech and the organ producing it are one and the same. Distantly related to the English word "language" through the shared metaphor of the tongue as the instrument of speech, though the roots themselves differ.
Cultural note
Russians use язык seamlessly in both senses: говорить на русском языке ("to speak in the Russian language") and прикусить язык ("to bite one's tongue," literally and figuratively — meaning to stop yourself from saying something). The dual meaning also shows up in the proverb «Язык до Киева доведёт» — "Your tongue will lead you all the way to Kyiv," meaning that if you're willing to ask for directions, you'll find your way anywhere. A classic false-friend trap: the word looks unrelated to English "language" (from Latin lingua, itself also meaning both tongue and language), yet the underlying metaphor is identical.