Annum is a form of the Latin noun annus meaning year, from which are derived words such as annual and annuity. Annum is the accusative singular of the 2nd declension masculine noun annus (nominative singular: this is the reference form of the word), anni (genitive singular and nominative plural). Thus it is synonymous with year.

In astronomy it is defined as 365.25 days (that is, the average length of a year in the Julian calendar) of 86,400 SI seconds each. 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 unit of area called the "are", but context is usually enough to disambiguate). In English, the abbreviation yr is still used informally but is deprecated in most scientific usage.
The Unified Code for Units of Measure disambiguates the varying symbologies of ISO 1000, ISO 2955 and ANSI X3.50 by using

Prefix multipliers

• per annum means "yearly". (The Latin preposition per takes the accusative case.)

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

• megannum (more correctly megannus, sometimes megaannum - and similarly for the other prefixes), 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 into the past or future. 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. In geology and palentology, the year is not so precise and varies depending on the author.

• gigannum, 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.

• terannum, 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. It is the same order of magnitude as the expected life span of a small red dwarf star.

• petannum, 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.

• exannum, 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 unit symbols

• bya - Formerly used for Ga (ago)

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

• gya - Formerly used for Ga (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 unit symbols are now deprecated in modern geophysics, and their use is controversial in modern geology. 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)