Periodic Limb Movement Disorder (PLMD), also called<unk> , is a sleep disorder where the patient moves limbs involuntarily during sleep and has symptoms or problems related to the movement.  If the patient moves limbs during sleep but there are no consequences, it is simply called periodic limb movements of sleep (PLMS).  Not all patients with PLMS have PLMD (in fact, most people with PLMS do not have a disorder requiring any treatment).
PLMS is diagnosed with the aid of a polysomnogram or PSG.  PLMD is diagnosed by first finding PLMS on a PSG, then integrating that information with a detailed history from the patient and/or bed partner.  PLMS can range from a small amount of movement in the ankles and toes, to wild flailing of all four limbs. These movements, which are more common in the legs than arms, occur for between 0.5 and 5 seconds, recurring at intervals of 5 to 90 seconds. A formal diagnosis of PLMS requires three periods during the night, lasting from a few minutes to an hour or more, each containing at least 30 movements followed by partial arousal or awakening.
PLMS can cause insomnia and daytime sleepiness. The incidence of this disorder increases with age. It is estimated to occur in 5% of people age 30 to 50 and in 44% of people over the age of 65 (which means it is a normal observation in most individuals). As many as 12.2% of patients suffering from insomnia and 3.5% of patients suffering from excessive daytime sleepiness may experience PLMS.
PLMS is related to restless leg syndrome (RLS) in that 80% of people with RLS also have PLMS. However, most people with PLMS do not experience RLS.
PLMS has made its way into popular culture, being referred to as "jimmy legs" and "jimmy arms" in an episode of Seinfeld, and has been known to cause couples to sleep in separate beds.  In both examples, one subject suffers from PLMS severely enough to disturb the sleep of the other subject.
Nocturnal myoclonus
Pseudoathetosis
Opisthotonus
Choreoathetosis
Hair-grooming syncope
Coccydynia
Athetosis
Bigeminal rhythm