Palsy is a medical condition that causes a person to lose control over their muscles, making it difficult or impossible to move certain parts of their body normally. The word comes from an old term for paralysis.

The most well-known type is cerebral palsy, which affects movement and posture because of damage to the developing brain before or shortly after birth. Someone with cerebral palsy might have trouble walking, speaking clearly, or controlling their hand movements, though the specific challenges vary greatly from person to person. Many people with cerebral palsy have normal or above-average intelligence, even though their bodies don't move the way they want them to.

Another type is Bell's palsy, which causes temporary weakness or paralysis on one side of the face. Someone with Bell's palsy might not be able to smile normally on one side or might have trouble closing one eye. Unlike cerebral palsy, Bell's palsy usually improves over time.

Historically, people used “palsy” to describe trembling or shaking, which is why you might see it in old books. Today, doctors use more specific medical terms, but “palsy” still appears in the names of certain conditions.