What does eccentricity mean?
Eccentricity is the quality of being unconventional, peculiar, or noticeably different from what is customary. It describes behavior, appearance, or habits that depart from social norms in a way others find striking, amusing, or charming rather than alarming — a man who keeps forty cats, or a professor who lectures in full Victorian dress, might be said to display great eccentricity. The word can also refer to a single such odd trait or act, as in "one of his many eccentricities." Beyond everyday use, eccentricity has a precise technical meaning in mathematics and astronomy, where it measures how much an ellipse, orbit, or other curve deviates from a perfect circle; a circle has zero eccentricity, while highly elongated comet orbits approach one. Derived from Greek roots meaning "out of center," the word carries a neutral to affectionate connotation in ordinary speech, celebrating harmless individuality rather than condemning deviation. It remains a useful term for discussing both human character and celestial mechanics.
The quality of being unconventional or peculiar; departure from what is customary or centered on the norm.
"The village tolerated her eccentricity because she was generous to everyone."
Uncountable when referring to the general quality; countable (plural "eccentricities") when referring to specific quirks or acts.
In mathematics and astronomy, a measure of how much an ellipse, orbit, or conic section deviates from circularity, ranging from 0 (a perfect circle) toward 1 (increasingly elongated).
"Mercury's orbit has the highest eccentricity of any planet in the solar system."
Strictly technical register; common in physics, astronomy, and engineering contexts.
The plural is used when referring to multiple individual odd traits or behaviors of a person or thing, rather than the general abstract quality.
"Over the years, his colleagues catalogued the eccentricities of his teaching style with quiet amusement."
In astronomy, Earth's orbital eccentricity of just 0.017 is why our seasons are caused by axial tilt — not by how close we get to the Sun.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of eccentricity
Eccentricity derives from the Medieval Latin "excentricus," meaning "out of the center," formed from Greek "ekkentros" (from "ek-" meaning "out of" plus "kentron" meaning "center"). The word entered English in the mid-16th century, first used in astronomical contexts to describe orbits not centered on Earth, before extending figuratively to describe unconventional people and behavior by the early 17th century. It shares its root with "eccentric," "eccentrically," "concentric," and "center" — all ultimately tracing back to the Greek "kentron."
Related word forms
How eccentricity is actually used
Generally neutral to affectionate in tone when describing people — often suggesting charm rather than derangement. The mathematical sense is technical and register-neutral. Countable when referring to individual quirks; uncountable when referring to the general quality.
Easily confused with eccentricity
"Excentricity" is an archaic or erroneous spelling; the standard modern spelling is "eccentricity."
"Eccentric" is primarily the adjective (or noun for a person) describing someone unconventional, while "eccentricity" is the noun denoting that quality itself.