1560s, in geometry, "plane figure contained by a right angle and a part of a circumference of a circle," from Latin segmentum "a strip or piece cut off, a cutting, strips of colored cloth," from secare "to cut" (from PIE root *sek- "to cut"), with euphonious alteration of -c- to -g- before -m-.
Latin segmentum was used in Medieval Latin as a geometry term, translating Greek tmema. The meaning "segmental portion of anything circular" is from 1640s; the general sense of "a division, section, part cut off or marked as separate from others" is from 1762.
1859 (implied in segmented), "divide or become divided into segments," in reference to cell division, from segment (n.). Transitive sense, "divide (something) into segments" is from 1872. Related: Segmenting.