- means "occurring every year".
- , usually abbreviated as '''ka,''' is a unit of time equal to one Thousand years.
- , usually abbreviated as '''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 flourished 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.
- , usually abbreviated as '''Ga''', is a unit of time equal to one Billion (109) years. It is commonly used in scientific disciplines such as Cosmology to signify extremely long time periods in the past. For example, the formation of the Earth occurred approximately ''4.55 Ga'' (4.55 billion years) ago. As is clear from the notation, ''billion'' as used here is on the Short Scale .
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