# Eccentricity

The variation of the outer diameter of a shaft surface when referenced to the true geometric centreline of the shaft. Often described as Out-of-roundness.

### Conic

The ratio of the length of the radius vector through a point on the conic to the distance of the point from the directrix.

### Ellipse

The eccentricity e of an ellipse can be computed by the formula:

where: a is the semimajor axis and b is the semiminor axis.

### Ellipse Linear Eccentricity

The ratio of the distance between the center and focus of an ellipse to its semimajor axis.

**See also: **Eccentricity Ratio, Ellipse, Off Centre, Runout.

**Subjects: ** Mechanical Engineering