A hypoid is the name given to a type of helical (spiral) gear. The main application of this is in the final drive of an automobile, where the direction of the drive carried by the propeller shaft has to be turned through 90 degrees in order to drive the rear wheels. Conventional straight-cut gears, with perpendicular teeth, are considered to be too noisy in use, and a normal spiral bevel does not always give sufficient contact area. The hypoid gear places the pinion off-axis to the crown wheel which allows the pinion to be larger in diameter. In a normal passenger car the pinion is always offset to the bottom of the crown wheel. This allows propeller shaft that drives the pinion to be lowered, so that the "hump" in the passenger compartment floor that it runs through doesn't intrude too much.
A hypoid gear incorporates some sliding and can be considered halfway between a straight-cut gear and a worm gear. Special gear oils are required for hypoid gears because the sliding action creates extreme pressure between the teeth.