What condition is described as sudden jerking of a single or group of muscles?

Myoclonus is characterized by sudden, involuntary jerking or twitching of a muscle or muscle groups. This type of movement can occur for a variety of reasons, including as a physiological response during certain stages of sleep or due to neurological disorders. The movements are often brief and can vary in intensity and frequency, which is a defining feature of myoclonus.

In contrast, Tourette's syndrome involves repetitive, non-rhythmic movements and vocalizations known as tics, but these are more complex and not limited to sudden jerking motions. Hemibalismus typically involves violent and flailing movements of one side of the body, generally due to a lesion on one side of the brain, and is distinct from the sudden muscle jerks seen in myoclonus. Tics, while they can include jerks, often encompass a broader range of movements and do not always feature the clarity or suddenness characteristic of myoclonus. Therefore, the defining aspect of myoclonus is the suddenness and singularity of the muscle jerks, making it the correct answer.