Monorchism

Monorchism is the state of having only one testicle within the scrotum.

Terminology

An individual having monorchism can be referred to as monorchid.

This can be due to one testicle:

Not descending into the scrotum during normal embryonic or fetal development, also known as undescended testis or cryptorchidism. In this case the testis is within the abdominal cavity, somewhere along the normal route of descent – most commonly, within the inguinal canal. Such a testis has an increased risk of malignancy.
Disappearing during development due to some intrauterine insult. This is thought to be most likely vascular, such as testicular torsion.
Not being created in the first place, known as congenital monorchism.
Being surgically removed through orchiectomy.
Being injured.

Archibald Douglas, 4th Earl of Douglas, magnate of the Kingdom of Scotland, and Peer of France. Lost in 1403, while fighting at the Battle of Shrewsbury.
Troy Bayliss, world superbike champion in 2001, 2006 and 2008. In 2007 he lost a testicle during a race at Donington Park.
Brian Foster, American mixed martial artist.
John Starks, American basketball player.
Paul Wood, English rugby league player who sustained a ruptured testicle during a match and subsequently had it removed.
Thurgood Marshall, United States Supreme Court Justice who injured a testicle during a fraternity event in university.
Dave England, who lost one of his testicles after suffering a double hernia during a snowboard accident in New Zealand in 1997.

Dara Ó Briain, Irish comedian related that he has a single testicle in a standup routine.

Although extremely rare, monorchism has been observed to be characteristic of some animal species, notably in beetles.