a

Reading level: easy

Estimated CEFR level: A1 — Beginner

Estimated from word frequency; not an official CEFR classification.

Definition

  1. noun 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. noun any of several fat-soluble vitamins essential for normal vision; prevents night blindness or inflammation or dryness of the eyes
  3. noun one of the four nucleotides used in building DNA; all four nucleotides have a common phosphate group and a sugar (ribose)

Etymology

The letter name is from Middle English ā, from Old French, ultimately from Latin ā. Use of the Latin letter in (Old) English displaced the futhorc letter ᚪ (a) beginning in the 7th century, and partially also ᚫ (æ).

In classic literature

Synonyms

angstrom, angstrom unit

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