The middle point on the posterior margin of the foramen magnum, opposite the basion. [G. opisthios, posterior]
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opis·thi·on ə-'pis-thē-.än n, pl -thia -thē-ə or -thions the median point of the posterior border of the foramen magnum
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opis·thi·on (o-pisґthe-on) [Gr. opisthion rear, posterior] [TA] a craniometric landmark located at the midpoint of the posterior border of the foramen magnum.Medical dictionary. 2011.