A

IPA (US):

 Dictionary entry overview: What does A mean? 

• A (noun)
  The noun A has 7 senses:

1. 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. any of several fat-soluble vitamins essential for normal vision; prevents night blindness or inflammation or dryness of the eyes

3. one of the four nucleotides used in building DNA; all four nucleotides have a common phosphate group and a sugar (ribose)

4. (biochemistry) purine base found in DNA and RNA; pairs with thymine in DNA and with uracil in RNA

5. the basic unit of electric current adopted under the Systeme International d'Unites

6. the 1st letter of the Roman alphabet

7. the blood group whose red cells carry the A antigen

  Familiarity information: A used as a noun is common.

 Dictionary entry details 

• A (noun)

Sense 1

Meaning:

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

Classified under:

Nouns denoting quantities and units of measure

Synonyms:

A; angstrom; angstrom unit

Hypernyms ("A" is a kind of...):

metric linear unit (a linear unit of distance in metric terms)

Meronyms (parts of "A"):

micromicron; picometer; picometre (a metric unit of length equal to one trillionth of a meter)

Holonyms ("A" is a part of...):

micromillimeter; micromillimetre; millimicron; nanometer; nanometre; nm (a metric unit of length equal to one billionth of a meter)

Sense 2

Meaning:

Any of several fat-soluble vitamins essential for normal vision; prevents night blindness or inflammation or dryness of the eyes

Classified under:

Nouns denoting substances

Synonyms:

A; antiophthalmic factor; axerophthol; vitamin A

Hypernyms ("A" is a kind of...):

fat-soluble vitamin (any vitamin that is soluble in fats)

Hyponyms (each of the following is a kind of "A"):

retinol; vitamin A1 (an unsaturated alcohol that occurs in marine fish-liver oils and is synthesized biologically from carotene)

dehydroretinol; vitamin A2 (a viscous alcohol that is less active in mammals than is vitamin A1)

Sense 3

Meaning:

One of the four nucleotides used in building DNA; all four nucleotides have a common phosphate group and a sugar (ribose)

Classified under:

Nouns denoting substances

Synonyms:

A; deoxyadenosine monophosphate

Hypernyms ("A" is a kind of...):

base; nucleotide (a phosphoric ester of a nucleoside; the basic structural unit of nucleic acids (DNA or RNA))

Sense 4

Meaning:

(biochemistry) purine base found in DNA and RNA; pairs with thymine in DNA and with uracil in RNA

Classified under:

Nouns denoting substances

Synonyms:

A; adenine

Hypernyms ("A" is a kind of...):

purine (any of several bases that are derivatives of purine)

Domain category:

biochemistry (the organic chemistry of compounds and processes occurring in organisms; the effort to understand biology within the context of chemistry)

Holonyms ("A" is a substance of...):

deoxyribonucleic acid; desoxyribonucleic acid; DNA ((biochemistry) a long linear polymer found in the nucleus of a cell and formed from nucleotides and shaped like a double helix; associated with the transmission of genetic information)

ribonucleic acid; RNA ((biochemistry) a long linear polymer of nucleotides found in the nucleus but mainly in the cytoplasm of a cell where it is associated with microsomes; it transmits genetic information from DNA to the cytoplasm and controls certain chemical processes in the cell)

Sense 5

Meaning:

The basic unit of electric current adopted under the Systeme International d'Unites

Classified under:

Nouns denoting quantities and units of measure

Synonyms:

A; amp; ampere

Context example:

a typical household circuit carries 15 to 50 amps

Hypernyms ("A" is a kind of...):

current unit (a measure of the amount of electric charge flowing past a circuit point at a specific time)

Meronyms (parts of "A"):

mA; milliampere (one thousandth of an ampere)

Holonyms ("A" is a part of...):

abamp; abampere (a unit of current equal to 10 amperes)

Sense 6

Meaning:

The 1st letter of the Roman alphabet

Classified under:

Nouns denoting communicative processes and contents

Hypernyms ("A" is a kind of...):

alphabetic character; letter; letter of the alphabet (the conventional characters of the alphabet used to represent speech)

Holonyms ("A" is a member of...):

Latin alphabet; Roman alphabet (the alphabet evolved by the ancient Romans which serves for writing most of the languages of western Europe)

Sense 7

Meaning:

The blood group whose red cells carry the A antigen

Classified under:

Nouns denoting body parts

Synonyms:

A; group A; type A

Hypernyms ("A" is a kind of...):

blood group; blood type (human blood cells (usually just the red blood cells) that have the same antigens)

 Context examples 

My dear Holmes, said I, this is too much. You would certainly have been burned, had you lived a few centuries ago.

(The Adventures of Sherlock Holmes, by Sir Arthur Conan Doyle)

It is a truth universally acknowledged, that a single man in possession of a good fortune, must be in want of a wife.

(Pride and Prejudice, by Jane Austen)

You may remember that a history of all the voyages made for purposes of discovery composed the whole of our good Uncle Thomas’ library.

(Frankenstein, by Mary Wollstonecraft Shelley)

But at the crest he saw a shallow valley, empty of life.

(Love of Life and Other Stories, by Jack London)

And yet there was the dead man and there the revolver bullet, which had mushroomed out, as soft-nosed bullets will, and so inflicted a wound which must have caused instantaneous death.

(The Return of Sherlock Holmes, by Sir Arthur Conan Doyle)

A reference source that provides meanings of words related to medical history.

(Medical History Dictionary, NCI Thesaurus)

“I am afraid, Watson, that I shall have to go,” said Holmes, as we sat down together to our breakfast one morning.

(The Memoirs of Sherlock Holmes, by Sir Arthur Conan Doyle)

The date and time a medical history was collected.

(Medical History Data Collection Date Time, NCI Thesaurus)

The day of the week a medical history data was collected.

(Medical History Collection Day, NCI Thesaurus)

“This is a handy cove,” says he at length; “and a pleasant sittyated grog-shop. Much company, mate?”

(Treasure Island, by Robert Louis Stevenson)