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Webster 1913 Edition

Noun.

[Gr. [GREEK];

χίλιοι

a thousand + [GREEK] angle.]

A plane figure of a thousand angles and sides.

Barlow.

Noun.

A plain figure of a thousand angles and sides.

chiliagon ‎(plural chiliagons)

(geometry) (rare) A polygon with a thousand vertices and a thousand edges.
- 1995: If Kant's claim is just that it is impossible to have a sensory experience of the absence of space, then by contrast it is not impossible to have a sensory experience of a chiliagon. — Lorne Falkenstein, Kant's Intuitionism (Toronto 2004, p. 198)

The term chiliagon is rarely used in mathematics, the term "1000-gon" being preferred.