A gag is an item or device designed to prevent speech, often as a restraint device to stop the subject from calling for help and keep its wearer silent. This is usually done by blocking the mouth, partially or completely, or attempting to prevent the tongue, lips, or jaw from moving in the normal patterns of speech. The more "effective" a gag appears to be, the more hazardous it is. For example, duct tape is a fairly effective method of keeping a persons mouth shut but can be hazardous if the subject cannot breathe freely through the nose. Gags are commonly depicted in soap operas and crime fiction, particularly in comics, novels, and films.

There are different types of gags, including:

• Cloth gag: A suitably sized piece of cloth pulled over the subjects mouth and tied at the back of their head. It is sometimes called the "detective gag" because many of its first appearances were in crime serials.

• Gag rule: A part of court proceedings and congressional proceedings.

• Gag bit: A special bit type used with horses.

In addition to its meaning as a restraint device, "gag" can also refer to the act of almost vomiting. It can also mean to prevent a person or organization from talking or writing about a particular subject. Finally, "gag" can refer to something that is funny or amusing, such as a joke or prank.