Pitch: The size of each tooth on the gear. 

Teeth: The number of teeth on the gear. 

Material: What the gear is made of, e.g. Aluminum, Bronze, Steel. 

Root Diameter: The distance from the base of a tooth on one side to the base of the opposing tooth on the other side of the gear. 

Pitch Diameter: The diameter from the point of engagement of one tooth one side to the base of the opposing tooth on the other side of the gear. 

Outside Diameter: The distance from the top of one tooth on one side of the gear to the top (farthest point) of the opposing tooth on the other side of the gear. 

Bore: The hole in the center of the gear. 

Hub ("B" Style): Gears with a protruding hub. 

No Hub ("A" Style):  Flat gears with no protruding hub. 

Face: The size of the tooth face. 

Line of Action: The angle at which the two gears engage. 

Pressure Angle:  The arc from the line of Action to the horizontal 90^o line dividing the two gears. 

Circular Tooth Thickness:  The thickness of one tooth at the point where the gear engages. 

Whole Depth:  The entire depth of the tooth cavity. 

Working Depth: The depth for that part of the tooth which engages the other gear. 

Center Distance: The distance from the center of the one gear to the center of the gear with which it is engaged.