Tremor

[Sarah]

A trembling; a shivering or shaking; a quivering or vibratorymotion; as, the tremor of a person who is weak, infirm, or old.He fell into an universal tremor of all his joints. Harvey.

[Britney]

An involuntary quivering movement.

[Jason Yan]

Tremors are involuntary, rhythmic, oscillatory movements of reciprocal, antagonistic muscle groups, typically involving the hands, head, face, vocal cords, trunk, or legs. Diagnosis is clinical. Treatment depends on the cause and type of tremor and may involve avoidance of triggers (physiologic), propranolol or primidone (essential), physical therapy (cerebellar), levodopa (parkinsonian), and possibly deep brain stimulation or thalamotomy (disabling and drug-refractory).
(See also Overview of Movement and Cerebellar Disorders.)
Tremor may be

Normal (physiologic)
Pathologic

Physiologic tremor, usually barely perceptible, becomes noticeable in many people during physical or mental stress.
Tremors vary in

Pattern of occurrence (eg, intermittent, constant)
Severity
Acuity (eg, gradual, abrupt)

The severity of tremor may not be related to the seriousness of the underlying disorder. For example, essential tremor is generally thought of as benign and should not shorten life, but symptoms can be disabling, and cerebellar degeneration has been detected in some neuropathologic studies.

[Agnes]

a slight shaking movement in a persons body, especially because of nervousness or excitement: