Definition of a in English:

a

  • 1(in travel timetables) arrives:

    ‘Penzance a 0915’
  • 2[in combination] (in units of measurement) atto- (10−18).

  • 3British (with reference to sporting fixtures) away:

    ‘March 15 Sheffield United (a)’
  • 4(used before a date) before:

    ‘a1200’

"a"
Physics
  • Acceleration.