The average time interval for a variable star to complete a cycle of its variations.

A family of elements with consecutive atomic numbers in the periodic table and with closely related properties; for example, chromium through copper.

A unit of geologic time constituting a subdivision of an era; the fundamental unit of the standard geologic time scale.

1. A number T such that f (x + T ) = f (x ) for all x, where f (x ) is a specified function of a real or complex variable.

2. The period of an element a of a group G is the smallest positive integer n such that aⁿ is the identity element; if there is no such integer, a is said to be of infinite period.

The time required for exponentially rising or falling neutron flux in a nuclear reactor to change by a factor of e (2.71828).

The duration of a single repetition of a cyclic phenomenon.