segmentn.[L. segmentum, fr. secare to cut, cut off: cf. F. segment. See Saw a cutting instrument.]1. One of the parts into which any body naturally separates or is divided; a part divided or cut off; a section; a portion; as, a segment of an orange; a segment of a compound or divided leaf.()2. (Geom.) A part cut off from a figure by a line or plane; especially, that part of a circle contained between a chord and an arc of that circle, or so much of the circle as is cut off by the chord; as, the segment acb in the Illustration.()3. (Mach.) A piece in the form of the sector of a circle, or part of a ring; as, the segment of a sectional fly wheel or flywheel rim.()4. (Biol.) One of the cells or division formed by segmentation, as in egg cleavage or in fissiparous cell formation.()Segment gear, a piece for receiving or communicating reciprocating motion from or to a cogwheel, consisting of a sector of a circular gear, or ring, having cogs on the periphery, or face. -- Segment of a line, the part of a line contained between two points on it. -- Segment of a sphere, the part of a sphere cut off by a plane, or included between two parallel planes. -- Ventral segment. (Acoustics) See Loor, n., 5.()v. i. (Biol.) To divide or separate into parts in growth; to undergo segmentation, or cleavage, as in the segmentation of the ovum.()