Phimosis is the inability to pull the foreskin back over the head of the penis (glans).
The head of penis is covered by a tissue called the foreskin. The foreskin is usually loose and slides easily over the head of the penis. This movement allows the child to urinate or to become fully erect (in adolescents). Sometimes, though, the foreskin is too tight. It can close over the glans and become unable.