Eccentric is from the Greek for out of the centre, as opposed to concentric, in the centre.

• In mathematics, eccentricity is a parameter associated with every conic section. It can be thought of as a measure of how much the conic section deviates from being circular.
• In astrodynamics, the eccentricity of an orbit can be calculated from orbital state vectors as an absolute value of eccentricity vector or using other methods based on orbital energy and angular momentum.
• In mechanical engineering, an eccentric is a wheel that rotates on an axle that is displaced from the focus of the circle described by the wheel —in other words, a mechanical motion that can operate either as a cam or a crank, depending upon how it is driven.
• In popular usage, eccentricity refers to unusual or odd behaviour on the part of a person, as opposed to being "normal". There is a saying that "If you're rich, you're eccentric; if you're poor, you're crazy." See also List of people widely considered to be eccentric.