Annum is a Latin noun meaning year. It is the accusative singular of the second declension masculine noun annus (nominative), anni (genitive) 2nd%20dec.htm" target="_blank">http://www.heathmount.org/annus_2nd%20dec.htm.

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 31-1 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 frequently used.

The Unified Code for Units of Measure disambiguates the symbology by using

• 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 (10 6 ) 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; "mya" 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 10 9 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 10 12 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 10 15 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 10 18 years (one quintillion on the short scale, one trillion on the long scale). The half-life of tungsten-180 is 1.8 Ea.