Something or someone that is strange or unusual.
Any of various angular distances.
A defect or malformation.
A failure of a classical symmetry due to quantum corrections.
Any event or measurement that is out of the ordinary regardless of whether it is exceptional or not.
A deviation from a rule or from what is regarded as normal; an outlier.
A person or thing whose nature or value is a mystery.
Used other than figuratively or idiomatically: see unknown, quantity.