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1) angstrom, angstrom unit, A: a metric unit of length equal to one ten billionth of a meter (or 0.0001 micron); used to specify wavelengths of electromagnetic radiation

2) vitamin A, antiophthalmic factor, axerophthol, A: any of several fat-soluble vitamins essential for normal vision; prevents night blindness or inflammation or dryness of the eyes

3) deoxyadenosine monophosphate, A: one of the four nucleotides used in building DNA; all four nucleotides have a common phosphate group and a sugar (ribose)

4) adenine, A: (biochemistry) purine base found in DNA and RNA; pairs with thymine in DNA and with uracil in RNA

5) ampere, amp, A: the basic unit of electric current adopted under the Systeme International d’Unites; “a typical household circuit carries 15 to 50 amps”

6) A, letter a: the 1st letter of the Roman alphabet

7) A, type A, group A: the blood group whose red cells carry the A antigen


“A” is used before a consonant sound like words that start with “d”, “t”, “h”, etc.
Example: a door, a human

“An” is used before a vowel sound like “an icicle”, ”
an honor”. Note that when a word begins with a vowel letter but is pronounced with a consonant sound, a is used. “a one”, “a union”. When an abbreviation begins with a consonant letter but is pronounced with a vowel sound, an is used. ▪ an FBI investigation ▪ an LCD display. When a word begins with h but the first syllable of the word is not given primary stress, both a and an are used. “a historic”, “an historic”.

1. — used before singular nouns when the person or thing is being mentioned for the first timeThere was a tree in the field. ▪ A man walked past him. ▪ I heard a shout. ▪ He bought a house, but this is not the house he bought. ▪ I ordered an apple and some cheese: I ate the apple but not the cheese.

2. a — used like one before number words like hundred, thousand, etc. ▪ a hundred and twenty people ▪ a million dollars ▪ a dozen doughnuts
b. — used like one before number words like third, fortieth, etc. ▪ This is a [=one] third the size of that.
c. — used like one before units of weight, measurement, etc. ▪ a pound or two [=one or two pounds] ▪ a week or two [=one or two weeks] ▪ a foot and a half of water = one and a half feet of water
d. – one single : even one — used in negative constructions ▪ They didn’t charge me a penny. [=they didn’t charge me anything at all] ▪ “Did she say anything about it?” “Not a word.”

3. — used before a word or phrase that indicates a type or class of person or thing ▪ My uncle is a plumber. ▪ Copper is a metal. ▪ She’s a very nice lady. — often used before noncount nouns that are modified by an adjective or phrase ▪ a torrential rainShe has a warmth that puts people at ease. ▪ I’ve always had a fondness for chocolate.

4. — used like any to refer in a general way to people or things ▪ A person who is sick can’t work well. [=people can’t work well if they are sick]

5. — used before a proper noun to indicate limited knowledge about the person or thing being mentioned ▪ A Mr. Smith [=a man named Mr. Smith] called to ask about the job. ▪ Among the towns of the area there is a Newton, I believe.

6. a — used before a proper noun that is acting as an example or typethe attractions of a Boston or a Cleveland ▪ His friends say he’s an Einstein in regard to science.
b. — used before the name of a day of the week to refer to one occurrence of it ▪ My birthday falls on a Tuesday this year.

7. a — used before the name of a person (such as a famous artist) when the name is being used to refer to something (such as a painting) created by that personThe museum recently purchased a Rembrandt. [=a painting by Rembrandt] ▪ My violin is a Stradivarius.
b — used before a family name to show that someone is a member of that family ▪ Did you know that she’s a Kennedy?

8 — used before a proper noun referring to a person or thing that has a particular quality ▪ A triumphant Ms. Jones greeted her supporters. [=Ms. Jones was triumphant when she greeted her supporters] ▪ We were met at the door by an embarrassed Mr. Brown. [=Mr. Brown was embarrassed when he met us at the door] ▪ We had a very mild January.

9 — used with words like bit and little to form phrases that describe quantity, amount, or degreeShe felt a bit tired. ▪ It’s getting a little late.

10 — used in phrases that describe how often something occurs, how fast something is going, etc. ▪ They meet twice a week. [=twice each week] ▪ The car was traveling at a hundred miles an [=per] hour.

noun a has 7 sense(s) (first 1 from tagged texts)

a (Wikipedia)
This article is about the letter of the alphabet. For the English indefinite article, see English articles. For other uses, see A (disambiguation).
For technical reasons, "A#" redirects here. For A-sharp, see A♯ (musical note).
Writing cursive forms of A

A (named a /ˈ/, plural aes) is the 1st letter and the first vowel in the ISO basic Latin alphabet. It is similar to the Ancient Greek letter alpha, from which it derives. The upper-case version consists of the two slanting sides of a triangle, crossed in the middle by a horizontal bar. The lower-case version can be written in two forms: the double-storey a and single-storey ɑ. The latter is commonly used in handwriting and fonts based on it, especially fonts intended to be read by children. It is also found in italic type.

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