A
semi-voluntary muscular
spasm which
afflicts some
people.
Twitches can be a result of many
factors, but are normally an ¨
escape¨ from
stress. They´re usually
harmless, but
annoying as hell. Some light
anti-depressants can
alleviate the problem.
Therapy also helps in some cases.
The most common forms involve abnormal eye movement, head-movements or movements in any other commonly used muscle. Some subtle forms also exist (and people don´t even realize they have it) like shaking the legs rhytmically under the table, playing with objects like pencils, pens, etc.
Animals also exhibit some peculiar forms of twitches, like the bulls, who make a characteristic leg movement when they´re ready to charge.
A very interesting book that can provide some (indirect) insight on the subject is ¨The Naked Ape¨ by Desmond Morris -- A good read, even if you don´t have twitches.