What does "expression" mean?

Additional senses

1. 2.Lively or vivid representation of meaning, sentiment, or feeling, etc.; significant and impressive indication, whether by language, appearance, or gesture; that manner or style which gives life and suggestive force to ideas and sentiments; as, he reads with expression; her performance on the piano has expression. The imitators of Shakespeare, fixing their attention on his wonderful power of expression, have directed their imitation to this. M. Arnold.
2. 3.That which is expressed by a countenance, a posture, a work of art, etc.; look, as indicative of thought or feeling. "The expression of an eye." Tennyson. It still wore the majesty of expression so conspicuous in his portraits by the inimitable pencil of Titian. Prescott.
3. 4.A form of words in which an idea or sentiment is conveyed; a mode of speech; a phrase; as, a common expression; an odd expression.
4. 5.(Math.) The representation of any quantity by its appropriate characters or signs. Past expression, Beyond expression, beyond the power of description. "Beyond expression bright." Milton.