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In plane geometry, octant is a circular sector obtained by division of the full circle into 8 equal parts, hence the name. It is a sector with the central angle of 45°.
Usage
Most commonly, octants are seen on the compass rose.
Traditionally wind direction is given as one of the 8 octants (N, NE, E, SE, S, SW, W, NW) because that is more precise than merely giving one of the 4 quadrants, and the wind vane typically does not have enough accuracy to bother with more precise indication.
The name of the instrument "octant" comes from the fact that it is based on 1/8th of the cirle.
References
Octant (plane geometry) Wikipedia(Text) CC BY-SA