acceleration

Short definition from Wiktionary

Noun

1. 1
   The act of accelerating, or the state of being accelerated; increase of motion or action; as opposed to retardation or deceleration.
2. 2
   The amount by which a speed or velocity increases (and so a scalar quantity or a vector quantity).
3. 3
   The change of velocity with respect to time (can include deceleration or changing direction).
4. 4
   The advancement of students at a rate that places them ahead of where they would be in the regular school curriculum.