Annum is one form of the Latin noun meaning year, not a form normally used for derivatives in modern languages: the accusative singular of the second declension masculine noun annus (nominative), anni (genitive singular and nominative plural).

As a unit of time, it is defined as exactly 365.25 days (that is, the average length of a year in the Julian calendar) of 86,400 SI seconds each, representing the duration of one revolution of the Earth around the Sun. Although there is no universally accepted symbol for the year, NIST SP811 and ISO 80000-3:2006 suggest the symbol a (in the International System of Units a is also the symbol for the are unit of area, but context is usually enough to disambiguate). In English, the deprecated abbreviation yr is still used informally.

The Unified Code for Units of Measure disambiguates the varying symbologies of ISO 1000, ISO 2955 and ANSI X3.50 by using

Multiples of an "annum"

• per annum means "yearly".

• kiloannum, usual symbol ka, is a unit of time equal to one thousand years.

• megaannum, usual symbol Ma, is a unit of time equal to one million (10⁶) years. It is commonly used in scientific disciplines such as geology, paleontology, and celestial mechanics to signify very long time periods in the past. For example, the dinosaur species Tyrannosaurus rex was abundant approximately 65 Ma (65 million years) ago (ago may not always be mentioned; if the quantity is specified while not explicitly discussing a duration, one can assume that "ago" is implied; the alternative but deprecated "mya" unit includes "ago" explicitly.). In astronomical applications, the year used is the Julian year of precisely 365.25 days.

• gigaannum, usual symbol Ga, is a unit of time equal to 10⁹ years (one billion on the short scale, one milliard on the long scale). It is commonly used in scientific disciplines such as cosmology and geology to signify extremely long time periods in the past. For example, the formation of the Earth occurred approximately 4.57 Ga (4.57 billion years) ago.

• teraannum, symbol Ta, is a unit of time equal to 10¹² years (one trillion on the short scale, one billion on the long scale). It is an extremely long unit of time, about 70 times as long as the age of the universe. The expected life span of a small red dwarf star.

• petaannum, symbol Pa, is a unit of time equal to 10¹⁵ years (one quadrillion on the short scale, one billiard on the long scale). The half-life of the nuclear isomer tantalum-180m is about 1 Pa.

• exaannum, usual symbol Ea, is a unit of time equal to 10¹⁸ years (one quintillion on the short scale, one trillion on the long scale). The half-life of tungsten-180 is 1.8 Ea.

Deprecated units

• bya - Formerly used for Ga (ago)

• byr - Formerly used for Ga (either elapsed or ago)

• mya - Formerly used for Ma (ago)

• myr - Formerly used for Ma (either elapsed or ago)

• tya (sometimes spelled kya) - formerly used for ka (ago)

• kyr - Formerly used for ka (either elapsed or ago)

These are deprecated units. Except for kyr they do not use accepted SI prefixes. Further, the suffixes ya and yr are not accepted SI units for time. However ya would be the symbol for the yoctoannum unit of time. 1 ya would be 10^-24 a which would be about of 3.15 x 10^-17 s.

See also

• Before Present

• Orders of magnitude (time)