# Flattening

**Flattening** is a measure of the compression of a circle or sphere along a diameter to form an ellipse or an ellipsoid of revolution (spheroid) respectively. Other terms used are **ellipticity**, or **oblateness**. The usual notation for flattening is and its definition in terms of the semi-axes and of the resulting ellipse or ellipsoid is

The *compression factor* is in each case; for the ellipse, this is also its aspect ratio.