(adjective) Not engaging in, or incapable of, motion.
Frequently used to refer to sperm; immotile sperm are a leading cause of male infertility, and can be caused by various genetic mutations (male sufferers of Kartagener's syndrome and immotile cilia syndrome, for instance, are infertile for this reason), infection by bacteria such as E. coli, and even excessive temperature. One popular (although only marginally effective) method of male contraception in some cultures is to dip the scrotum in uncomfortably warm water for about half an hour just prior to intercourse.