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Webster 1913 Edition

Noun.

[Cf. F.

trisection

The division of a thing into three parts, Specifically:

(Geom.)

the division of an angle into three equal parts.

Noun.

[L. tres and sectio, a cutting.] The division of a thing into three parts; particularly in geometry, the division of an angle into three equal parts.

trisection ‎(plural trisections)

A traditional mathematical problem was to achieve this using straight edge and compass (impossible)

division of angle into three