The Hawaiian word kapu best translates as "forbidden". In ancient Hawaii, kapu refers to the ancient system of laws and regulations. An offense that was kapu was often a corporal offense, but also often denoted a threat to spiritual power, or theft of mana. Kapus were strictly enforced. Breaking one, even unintentionally, often meant immediate death.

Most famous are the restrictions placed upon contact with chiefs, but these also apply to all people of known spiritual power. It was kapu to enter a chief's personal area, to come in contact with his hair or fingernail clippings, to look directly at him and to be in sight of him with a head higher than his. Wearing red and yellow feathers (a sign of royalty) was kapu, unless you were of the highest rank. Places that are kapu are often symbolized by two crossed staffs, each with a white ball atop.

The kapu system also governed contact between men and women. In particular, men and women could not eat meals together. Furthermore, certain foods such as pork, bananas, and coconuts were considered kapu to women.

The kapu system was used in Hawaii until 1819, when King Kamehameha II abolished it by symbolically sharing a meal of forbidden foods with the women of his court.

In modern usage in Hawai`i, KAPU is often substituted for the phrase "No Trespassing" on private property signage.

Tapu and tabu are spellings for the related concept in other Polynesian cultures. The concept of taboo in the English language derives from the Polynesian concept.