a

a
I
 ( or   A) /ˈeı/  noun  ,  pl   a's  or   as  or   A's  or   As
  1 : the first letter of the English alphabet
   [  count  ] a word that begins with an a
   [  noncount  ] a word that begins with a
  2 : a musical note or key referred to by the letter A : the sixth tone of a C-major scale
   [  count  ] play/sing an A
   [  noncount  ] a song in the key of A
  3 [  count  ] : a grade that is given to a student for doing excellent work
   She got an A on the exam.
   He's an A student. [=a student who gets A's for his schoolwork]
  4 [  noncount  ]
  — used to refer to the first of two or more people, places, or things that are being considered
   If A, B, and C divide the coins equally, how many does each person get?
  an A for effort — see effort
  exhibit A — see exhibit, 2
  from A to Z
   : including everything
    The book is titled “Home Repairs From A to Z.”
  from (point) A to (point) B
   : from one place to another
    I don't care about the scenery. I'm only interested in getting from A to B.
————————
II
 /ə, ˈeı/ ( or   an) /ən, ˈæn/  indefinite article  
  ◇ A is used before a consonant sound.
   a door
   a human
  An is used before a vowel sound.
   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 time
   There 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
  2 b
  — used like one before number words like third, fortieth, etc.
   This is a [=one] third the size of that.
  2 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
  2 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 rain
   She 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 type
   the attractions of a Boston or a Cleveland
   His friends say he's an Einstein in regard to science.
  6 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 person
   The museum recently purchased a Rembrandt. [=a painting by Rembrandt]
   My violin is a Stradivarius.
  7 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 degree
   She 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.

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