Monorchism (AKA monorchidism) is the state of having only one testicle, or only one testicle which has descended into the scrotum. An individual with monorchism is said to be monorchid (or, more rarely, monorchidic).
Monorchism can be due to abnormal development in the embryo or fetus, or may be the result of an operation. It is most often used in reference to humans, but I first came across it as a medical term applied to a cat.
It comes from the Greek word monochis, mono- meaning 'one', and orchis meaning 'testicle'.
If you liked this word, you might also like cryptorchidism, meaning without testicles.