With the gears found in RC cars and trucks, pitch is measured by the number of teeth in the gear. The spur gear is turned by the pinion gear (electric) or clutch bell (nitro) and provides power to the drivetrain. When matching gears, each must have the same pitch so the that gears mesh properly and turn together. A gear may be described as 48p or 32p, etc. In general (with variables) more teeth (higher pitch) provides more high-end speed. Lower pitch gives more low-end torque.
With boat props, the pitch is a measure of the how many inches a boat's propeller moves through the water in one revolution. And that's determined by the angle or pitch of the individual blades on the propeller. An RC boat prop may be described by its diameter and pitch (in inches or millimeters). For example, a prop may be 1.6-inch diameter by 2.5-inch pitch -- that is, the prop is 1.6 inches in diameter but the pitch or angle of the blades causes it to travel 2.5 inches through the water with one full turn of the prop. In general, a higher pitch provides greater speed but acceleration is slower. Finding the right diameter/pitch prop is a balancing act between power (torque) and speed.
On RC helicopters, pitch refers to the angle of the main rotor blades to the fuselage. The collective (also known as main blade pitch) controls that pitch or angle and allows for the up and down movement of the helicopter.