A condition in which the small size of the preputial opening of the foreskin (specifically the frenar band) prevents the skin from retracting behind the glans.

Phimosis can only be diagnosed after the foreskin becomes mobile and retractable. Some males don't have a fully retractable foreskin until puberty.

Congenital Phimosis is normal in infancy, childhood, and in some cases into the teens. If phimosis continues into adulthood, it can be resolved with gentle stretching of the opening. (Also called Preputial Stenosis).

Image Source: http://en.wikipedia.org/wiki/File:Phimosis_Penis.jpg

Pathological Phimosis is often an acquired dermatological condition affecting the foreskin and glans. A whitish ring of hardened tissue typically forms near the tip of the intact foreskin, preventing retraction.  This is a chronic condition and can be difficult to treat.  Other causes include scarring caused by forcible retraction of the foreskin and balanitis.  Depending on the cause, antibiotics or steriodal creams can be used to treat the condition. In rare cases circumcision is indicated, typically when the frenar band is hardened and the tissue develops fissures.