Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. Greatness of size; magnitude.
 n. Fullness; copiousness.
 n. Breadth or range, as of intelligence.
 n. Astronomy The angular distance along the horizon from true east or west to the intersection of the vertical circle of a celestial body with the horizon.
 n. Physics The maximum absolute value of a periodically varying quantity.
 n. Mathematics The maximum absolute value of a periodic curve measured along its vertical axis.
 n. Mathematics The angle made with the positive horizontal axis by the vector representation of a complex number.
 n. Electronics The maximum absolute value reached by a voltage or current waveform.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. The measure of something's size, especially in terms of width or breadth; largeness, magnitude.
 n. The maximum absolute value of the vertical component of a curve or function, especially one that is periodic.
 n. The maximum absolute value of some quantity that varies.
from the GNU version of the Collaborative International Dictionary of English
 n. State of being ample; extent of surface or space; largeness of dimensions; size.
 n. Largeness, in a figurative sense; breadth; abundance; fullness.
 n. Of extent of capacity or intellectual powers.
 n. Of extent of means or resources.
 n.
 n. The arc of the horizon between the true east or west point and the center of the sun, or a star, at its rising or setting. At the rising, the amplitude is eastern or ortive: at the setting, it is western, occiduous, or occasive. It is also northern or southern, when north or south of the equator.
 n. The arc of the horizon between the true east or west point and the foot of the vertical circle passing through any star or object.
 n. The horizontal line which measures the distance to which a projectile is thrown; the range.
 n. The extent of a movement measured from the starting point or position of equilibrium;  applied especially to vibratory movements.
 n. An angle upon which the value of some function depends;  a term used more especially in connection with elliptic functions.
from The Century Dictionary and Cyclopedia
 n. The state or quality of being ample in size; extension in space, especially breadth or width; largeness; extent.
 n. The state of being ample in amount; breadth in a figurative sense; fullness; abundance; copiousness.
 n. Largeness of mind; extent of mental capacity or of intellectual power; breadth of thought.
 n. In mathematics: In algebra, a positive real number multiplied by a root of unity. The positive real number is said to be the amplitude of the product. In elliptic integrals, the limit of integration when the integral is expressed in the usual trigonometric form.
 n. In astronomy, the arc of the horizon intercepted between the east or west point and the center of the sun or of a star at its rising or setting.
 n. the angle whose tangent is the hyperbolic sine of u.
 n. In function theory, if x =
ξ + in, let the polar coördinates of (ξ, η ) beρ, θ , then any one of the anglesθ orθ + 2nπ , where n is any positive or negative integer, may be called the amplitude of x.  n. In meteorology, the range or difference between the maximum and minimum values of the temperature, pressure, or other meteorological element within a definite time, such as a day, a month, or a year.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 n. (physics) the maximum displacement of a periodic wave
 n. the property of copious abundance
 n. greatness of magnitude
Etymologies
Examples

The term amplitude, thus used (by English writers also), is an old one in astronomical terminology.

Each whole number increase (ie from 5.0 to 6.0) represents a tenfold increase in amplitude (or about a 30fold increase in energy).

Annual temperature amplitude is 10,6°C (from 15°C to 35°C in summer and from 0°C to 15°C in winter), being February the hottest month.

However, if it is, it leads me to suspect that discrimination functions like a dampened oscillator (going back and forth and gradually decreasing in amplitude until it finally diminishes to an unnoticable level).

The seismic waves of a magnitude 6 earthquake are 10 times greater in amplitude than those of a magnitude 5 earthquake.

From a deified ancestor he would become a local god, like the Hebrew Jehovah as opposed to Chemosh of Moab; the name would gain amplitude by long time and distant travel, and the old island chieftain would end in becoming the

That amplitude is e to the times the action for the path.

This tells you the amplitude of fluctuations as a function of scale; if n = 1, the amplitude is the same on all scales.

The amplitude is the width of swing of the individual particles of the waves.

The amplitude is a measure of the power of the wave, the degree to which it disturbs the medium in which it propagates.
newyuppie commented on the word amplitude
Amplitude is a word sensory experts use to describe flavors that are well blended and balanced, that "bloom" in the mouth. (Gladwell)
December 7, 2009