The term cilice traditionally refers to the hair shirt, a garment or undergarment made of coarse cloth or animal hair. The word is pronounced "SILL-iss" (IPA ), and derives from the Latin cilicium, a covering made of goat's hair from Cilicia, a Roman province in south-east Asia Minor.

Such garments were worn at various times in the history of the Christian faith, for the purposes of the mortification of the flesh. Apart from the itchiness of the shirt's coarse texture, when worn continuously it formed a breeding-ground for lice, which would have the effect of increasing the discomfort. Saints and monks are often reported to have worn one, and rich men sometimes wore one under their fine clothes as penance for adorning themselves, something forbidden in the Bible. For instance, in the Holbein portrait of Thomas More, a small portion of a hair shirt appears to be visible near the wrists and at the neck under the fine robes of the Lord Chancellor.

In more recent times the word has come to refer to a spiked metal belt or chain worn strapped tight around the upper thigh. This practice has existed in various parts of the Roman Catholic Church, but has become associated in the twentieth century with the Catholic personal prelature known as Opus Dei.