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From The Collaborative International Dictionary of English v.0.48:
Symmetry \Sym"me*try\, n. [L. symmetria, Gr. ?; sy`n with, together + ? a measure: cf. F. sym['e]trie. See Syn-, and Meter rhythm.] 1. A due proportion of the several parts of a body to each other; adaptation of the form or dimensions of the several parts of a thing to each other; the union and conformity of the members of a work to the whole. [1913 Webster] 2. (Biol.) The law of likeness; similarity of structure; regularity in form and arrangement; orderly and similar distribution of parts, such that an animal may be divided into parts which are structurally symmetrical. [1913 Webster] Note: Bilateral symmetry, or two-sidedness, in vertebrates, etc., is that in which the body can be divided into symmetrical halves by a vertical plane passing through the middle; radial symmetry, as in echinoderms, is that in which the individual parts are arranged symmetrically around a central axis; serial symmetry, or zonal symmetry, as in earthworms, is that in which the segments or metameres of the body are disposed in a zonal manner one after the other in a longitudinal axis. This last is sometimes called metamerism. [1913 Webster] 3. (Bot.) (a) Equality in the number of parts of the successive circles in a flower. (b) Likeness in the form and size of floral organs of the same kind; regularity. [1913 Webster] Axis of symmetry. (Geom.) See under Axis. Respective symmetry, that disposition of parts in which only the opposite sides are equal to each other. [1913 Webster]