Definition

1. Indefinite Article:

   • Used before singular nouns to refer to a non-specific person or thing for the first time, or to indicate one single member of a class. It is used before consonant sounds.
   • Example: I saw a dog. (This introduces a dog that has not been mentioned before.)

2. Noun:

   • The first letter of the English and Roman alphabet.
   • A grade or score indicating the highest level of quality.
   • In music, the sixth note of the diatonic scale of C major.

Usage Examples

• Indefinite Article:

  • She is a doctor. (She belongs to the class of people who are doctors.)
  • I need a pen. (Any single, non-specific pen.)
  • He earns fifty dollars a day. (Per day.)

• Noun:

  • The word "apple" starts with an 'a'. (Referring to the letter.)
  • He got an A on his test. (Referring to the top grade.)
  • The orchestra tuned to the note A. (Referring to the musical pitch.)

Advanced Usage

• "A" as a prefix: Can mean "not" or "without" in words of Greek origin.

  • Atypical means not typical.
  • Amoral means not concerned with morality.

• "From A to Z": Covering everything completely; from beginning to end.

  • She knows the subject from A to Z.

• "A-1" or "A-one": Of the best quality; first-rate.

  • The service at the hotel was A-1.

Variants and Related Words

• An: The form of the indefinite article used before vowel sounds (e.g., , ).
• A-: A prefix, as in , meaning not social.

Synonyms

• One (when used to indicate a single item).
• Any (when used to indicate a non-specific item).
• Per (when used in expressions of rate).

Related Phrases (Phrasal Verbs)

• Not know A from B: To be completely ignorant.
  • He doesn't know A from B when it comes to fixing cars.

Related Idioms

• An A for effort: Recognition for trying hard, even if the result isn't perfect.
  • Your project had problems, but I give you an A for effort.

Noun

1. the blood group whose red cells carry the A antigen
2. the 1st letter of the Roman alphabet
3. the basic unit of electric current adopted under the Systeme International d'Unites
   • a typical household circuit carries 15 to 50 amps
4. (biochemistry) purine base found in DNA and RNA; pairs with thymine in DNA and with uracil in RNA
5. one of the four nucleotides used in building DNA; all four nucleotides have a common phosphate group and a sugar (ribose)
6. any of several fat-soluble vitamins essential for normal vision; prevents night blindness or inflammation or dryness of the eyes
7. a metric unit of length equal to one ten billionth of a meter (or 0.0001 micron); used to specify wavelengths of electromagnetic radiation