bya or b.y.a. is an

abbreviation An abbreviation () is a shortened form of a word or phrase, by any method including shortening (linguistics), shortening, contraction (grammar), contraction, initialism (which includes acronym), or crasis. An abbreviation may be a shortened for ...

for "

billion Billion is a word for a large number, and it has two distinct definitions: * 1,000,000,000, i.e. one thousand million, or (ten to the ninth power), as defined on the short scale. This is now the most common sense of the word in all varieties of ...

years ago". It is commonly used as a

unit of time A unit of time is any particular time interval, used as a standard way of measuring or expressing duration. The base unit of time in the International System of Units (SI), and by extension most of the Western world, is the second, defined as ...

to denote length of time before the present in 10⁹ years. This

initialism An acronym is a type of abbreviation consisting of a phrase whose only pronounced elements are the initial letters or initial sounds of words inside that phrase. Acronyms are often spelled with the initial letter of each word in all caps wi ...

is often used in the sciences of

astronomy Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...

geology Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...

, and

paleontology Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure ge ...

. The "billion" in bya is the 10⁹ "billion" of the short scale of the

U.S. The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 states and a federal capital district, Washington, D.C. The 48 contiguous ...

,Gale, Joseph (2009). ''Astrobiology of Earth: The Emergence''. Oxford: Oxford University Press, p
21.
/ref> not the long-scale 10¹² "billion" of some

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

an usage. Billion by this convention (10⁹) is often called a "thousand million" in the UK and a "milliard" in some other countries.Chartrand, Mark (2004). ''Satellite Communications for the Nonspecialist''. New York: SPIE Press, p
73.
/ref> For this reason, there is potential for some confusion, and some scientists prefer the unit Gya, while others prefer Ga (Giga-annum), however, bya remains in more widespread use. In 1974, the UK switched from the long scale to the short scale. Related units are mya ("million years ago"), and byr ("billion years"). These are traditionally written in

lowercase Letter case is the distinction between the letters that are in larger uppercase or capitals (more formally ''majuscule'') and smaller lowercase (more formally '' minuscule'') in the written representation of certain languages. The writing system ...

. Ga or Gya has a

capitalized Capitalization ( North American spelling; also British spelling in Oxford) or capitalisation (Commonwealth English; all other meanings) is writing a word with its first letter as a capital letter (uppercase letter) and the remaining letters in ...

first letter instead.

References

Units of time Chronology Metrology {{time-stub

See also

References