portion

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por·tion

(pōr'shŭn),
Part or division.

por·tion

(pōr'shŭn)
Part or division.

segment of a bifocal lens (seg) 

An area of a bifocal lens of a power different to that of the main portion. Syn. portion. There is the distance portion, which has the correction for distance vision, and the near or reading portion, which has the correction for near vision. See intermediate portion.