CHILIAGON, in Geometry, a regular plane figure of a thousand sides and angles. Though the imagination cannot form the idea of such a figure, yet we may have a very clear notion of it in the mind, and can easily demonstrate that the sum of all its angles is equal to 1996 right angles; for the internal angles of every plane figure are equal to twice as many right angles as the figure has sides, except the four which are about the centre of the figure, and hence it may be resolved into as many triangles as it has sides.