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 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 ».
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