Expression, n.
Etymology: [L. expressio. cf. F. expression.]
1. The act of expressing; the act of forcing out by pressure; as, the expression of juices or oils; also, of extorting or eliciting; as, a forcible expression of truth.
2. The act of declaring or signifying; declaration; utterance; as, an expression of the public will. With this tone of philosophy were mingled expressions of sympathy. Prescott.
3. 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.
4. 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 more senses
5. 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.
6. (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.
Shades of meaning
Fernald has no entry of its own for expression; he lists it under air, diction, language, term.
Where he writes about it:
Expression and look especially refer to the face. — from air
Expression is oftenest applied to that which is habitual; as, he has a pleasant expression of countenance; look may be momentary; as, a look of dismay passed over his face. — from air
An author's diction is strictly his choice and use of words, with no special reference to thought; expression regards the words simply as the vehicle of the thought. — from diction
Style includes diction, expression, rhetorical figures such as metaphor and simile, the effect of an author's prevailing tone of thought, of his personal traits--in short, all that makes up the clothing of thought in words; thus, we speak of a figurative style, a frigid or an argumentative style, etc., or of the style of Macaulay, Prescott, or others. — from diction
Language (F. langage < L. lingua, the tongue) signified originally expression of thought by spoken words, but now in its widest sense it signifies expression of thought by any means; as, the language of the eyes, the language of flowers. — from language
Thus, in general use term is more restricted than word, expression, or phrase; a term is a word that limits meaning to a fixed point of statement or to a special class of subjects, as when we speak of the definition of terms, that is of the key-words in any discussion; or we say, that is a legal or scientific term. — from term