Real Dictionary

gear
countable noun: The gears on a machine or vehicle are a device for changing the rate at which energy is changed into motion.
passive verb: If someone or something is geared to or towards a particular purpose, they are organized or designed in order to achieve that purpose.
uncountable noun: Gear means clothing.
uncountable noun: Some people refer to illegal drugs, especially heroin, as gear.
uncountable noun: The gear involved in a particular activity is the equipment or special clothing that you use.

gear in American English
the clothing and equipment of a soldier, knight, etc.
noun: a part, as a disk, wheel, or section of a shaft, having cut teeth of such form, size, and spacing that they mesh with teeth in another part to transmit or receive force and motion

gear in British English
noun: a toothed wheel that engages with another toothed wheel or with a rack in order to change the speed or direction of transmitted motion

gear in Mechanical Engineering
noun: A gear is a wheel with teeth that engage with the teeth of another wheel in order to change the speed or direction of transmitted rotational motion.