The formula for calculating eccentricity is e = c/a. In this formula, "e" refers to the eccentricity, "a" refers to the distance between the vertex and the center and "c" refers to the distance between the focus of the ellipse and the center.
In general, the term eccentric refers to something being off center. Mathematicians calculate eccentricity to determine how close the resemblance is between a conic section and a circle. A circle's eccentricity is always zero and with an ellipse, the eccentricity is less than one, but greater than zero. The eccentricity of two conic sections must be equal for them to be similar.