General English


  • (written as A)
    Class of stars with surface temperature of 7500–10000°, and strong spectral lines due to hydrogen. Examples include Sirius, Deneb, Vega and other bright stars. A stars tend to be white or bluish in colour. Apparent in the spectra of A stars are ionised metals like magnesium and calcium.


  • noun
    (written as A)
    letters that show how reliable a particular share, bond or company is considered to be


  • (written as A:)
    used in some operating systems to denote the first disk drive on the system
  • symbol
    (written as A)
    the hexadecimal equivalent of the decimal number 10
  • symbol forangstrom
    (written as )


  • noun
    (written as A)
    letters indicating that a share or bond or bank has a certain rating for reliability. The AAA rating (called the triple A rating) is given by standard & Poor’s or by Moody’s Investors Service, and indicates a very high level of reliability for a corporate or municipal bond in the USA.


  • (written as A-)
    symbol A minus.
  • symbol foracceleration
  • symbol forgain
    (written as A)
  • symbol formass number or nucleon number
    (written as A)
  • symbol foratto-
  • symbol foryear
  • A metric prefix representing 10-18. For example, 1 atto-volt is equal to 10-18 volt. Its abbreviation is a.


  • noun
    (written as A)
    a human blood type of the ABO system, containing the A antigen. Someone with this type of blood can donate to people of the same group or of the AB group, and can receive blood from people with this type or with type O.

Media Studies

  • symbol foramp
    (written as A)

General Science

  • prefix one quintillionth, 10–18.

Origin & History of “a”

The indefinite article in English is ultimately identical with the word one (as is the case, even more obviously, in other European languages – French un, German ein, and so on). The ancestor of both a(n) and one was ān, with a long vowel, but in the Old English period it was chiefly used for the numeral; where we would use a(n), the Anglo-Saxons tended not to use an article at all. Ān begins to emerge as the indefinite article in the middle of the 12th century, and it was not long before, in that relatively unemphatic linguistic environment, its vowel became weakened and shortened, giving an. And at about the same time the distinction between an and a began to develop, although this was a slow process; until 1300 an was still often used before consonants, and right up to 1600 and beyond it was common before all words beginning with h, such as house.