acceleration

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The act of accelerating.
• noun The process of being accelerated.
• noun Physics The rate of change of velocity with respect to time.

from The Century Dictionary.

• noun The act of accelerating, or the state of being accelerated: as—
• noun In mech., the rate of change of the velocity of a moving body; that is, the increment of velocity (in any direction) in the unit of time which would result were the rate of change to continue uniform for that length of time. The acceleration is said to be uniform if the body gains the same velocity in any constant direction in equal successive portions of time, no matter how small these portions may be taken. A constant force produces uniform acceleration in all cases; but it is sometimes convenient to substitute for some of the forces fictitious “constraints.” Thus, gravity (which near the earth's surface is sensibly a constant force) gives a falling body uniformly accelerated motion when the effect of the atmospheric resistance is eliminated; in this case the increment of velocity in each second, which is a little more than 32 feet, is called the acceleration of gravity, and in mechanical formulas is denoted by the letter g. When the velocity of a moving body continually diminishes, the acceleration is termed minus or negative, and the motion is said to be retarded; this is illustrated by the case of a ball thrown upward, the upward component of the velocity of which diminishes at the rate of 32 feet a second. Similarly, the force of friction which resists the motion of a sliding body is said to give it minus or negative acceleration.
• noun The shortening of the time between the present and the happening of any future event; specifically, in law, the shortening of the time before the vesting of a person with the possession of an expected interest. In physiology and pathology, increased activity of the functions of the body, particularly of the circulation of the fluids.
• noun In biology, the supposed acquisition of new characters by adults, and their inheritance by descendants at earlier and earlier stages of their life; tachygenesis (which see).

from the GNU version of the Collaborative International Dictionary of English.

• noun The act of accelerating, or the state of being accelerated; increase of motion or action; ; -- opposed to retardation.
• noun the increase of the moon's mean motion in its orbit, in consequence of which its period of revolution is now shorter than in ancient times.
• noun See Priming of the tides, under Priming.
• noun the amount by which their apparent diurnal motion exceeds that of the sun, in consequence of which they daily come to the meridian of any place about three minutes fifty-six seconds of solar time earlier than on the day preceding.
• noun the increasing velocity of their motion, in proceeding from the apogee to the perigee of their orbits.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun uncountable The act of accelerating, or the state of being accelerated; increase of motion or action; as opposed to retardation or deceleration.
• noun countable The amount by which a speed or velocity increases (and so a scalar quantity or a vector quantity).
• noun physics The change of velocity with respect to time (can include deceleration or changing direction).

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the act of accelerating; increasing the speed
• noun (physics) a rate of increase of velocity
• noun an increase in rate of change