Pilonidal - (Latin) Nest of hairs, containing an accumulation of hairs.

Sinus or cyst – (When referring to pilonidal disease) A cavity or pocket beneath the skin, connected by a narrow tract to a small opening at the skin surface (pit).

Pilonidal sinus - A hair-containing pocket under the skin (subcutaneous). Usually occurs in the cleft between the buttocks (sacrococcygeal area, natal cleft, tailbone area)

Pilonidal fistula – A subcutaneous Inflammatory tract originating in the midline pilonidal cavity and gradually extending up toward the lower back, laterally toward the buttocks or distally toward the anal region, creating secondary exit wounds in these regions.

* Note: pilonidal disease may include one or several pits and hair-containing pockets with one or more fistulas or without a fistula. 

* Note: Terms like pilonidal sinus, cyst or fistula are frequently used interchangeably by physicians to describe the same pilonidal disease, regardless of their exact meaning.

Pilonidal abscess - Acute infection within a pilonidal cavity forming a painful pus-containing bulge. Pilonidal abscess requires urgent treatment and drainage.

Ingrown hair – There is no agreement yet on the mechanism of pilonidal disease formation. The entrapped hair in the natal cleft is usually a free, shed hair derived from any region of the body (usually from the scalp). Some believe that locally ingrown hairs may also occasionally be the cause.

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