anomaly

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Deviation or departure from the normal or common order, form, or rule.
• noun One that is peculiar, irregular, abnormal, or difficult to classify.
• noun Astronomy The angular deviation, as observed from the sun, of a planet from its perihelion.

from The Century Dictionary.

• noun In meteorology, the amount by which a given observed quantity is greater or less than an assumed ideal or normal value; a departure.
• noun Deviation from the common rule or analogy; something abnormal or irregular.
• noun In astronomy, an angular quantity defining the position of a point in a planetary orbit, taken to increase in the direction of planetary motion.
• noun In music, a small deviation from a perfect interval in tuning instruments with fixed notes; a temperament.
• noun In mod. astron., the angle at the sun between perihelion and the place of a planet.

from the GNU version of the Collaborative International Dictionary of English.

• noun Deviation from the common rule; an irregularity; anything anomalous.
• noun The angular distance of a planet from its perihelion, as seen from the sun. This is the true anomaly. The eccentric anomaly is a corresponding angle at the center of the elliptic orbit of the planet. The mean anomaly is what the anomaly would be if the planet's angular motion were uniform.
• noun The angle measuring apparent irregularities in the motion of a planet.
• noun (Nat. Hist.) Any deviation from the essential characteristics of a specific type.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A deviation from a rule or from what is regarded as normal.
• noun Something or someone that is strange or unusual.
• noun sciences Any event or measurement that is out of the ordinary regardless of whether it is exceptional or not.
• noun astronomy Any of various angular distances.
• noun biology A defect or malformation.
• noun quantum mechanics A failure of a classical symmetry due to quantum corrections.
• noun dated An irregularity or disproportion.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (astronomy) position of a planet as defined by its angular distance from its perihelion (as observed from the sun)
• noun deviation from the normal or common order or form or rule
• noun a person who is unusual