tremor

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

• noun A shaking or vibrating movement, as of the earth.
• noun An involuntary trembling or quivering, as of the hands.
• noun A state or feeling of nervousness, worry, or excitement.
• noun A tremulous sound; a quaver.

from The Century Dictionary.

• noun In colonial furniture, a sort of hood or head-piece found on high cupboards and the like: probably derived from a French type.
• noun A shaking or quivering caused by some external impulse; a close succession of short vibratory or modulatory movements; a state of trembling in a living object or substance: as, the tremor of the aspen-leaf.
• noun An involuntary or convulsive muscular shaking, quaking, or quivering, as from weakness, disorder, or emotion.
• noun A trembling, quivering, or quavering quality or effect: as, a tremor of light.
• noun Synonyms Trepidation, Emotion, etc. (see agitation), quiver, quivering, quaking. See trepidation.

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

• noun A trembling; a shivering or shaking; a quivering or vibratory motion.

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

• noun A shake, quiver, or vibration.
• noun An earthquake.
• verb To shake or quiver excessively and rapidly or involuntarily; to tremble.

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

• noun an involuntary vibration (as if from illness or fear)
• verb shake with seismic vibrations
• noun shaking or trembling (usually resulting from weakness or stress or disease)
• noun a small earthquake