Metric Gears
Metric gears are defined by the module system, as follows: 

Module
The length in mm of the pitch circle diameter per tooth. 
MOD = PCD / N 

Number of Teeth
The number of teeth on the gear. 
N = PCD / MOD 

Pitch Circle Diameter
The diameter of the pitch circle. 
PCD = N x MOD 

Outside Diameter
The outside diameter of the gear. 
OD = (N + 2) x MOD 

Center Distance
The distance between the axes of two gears in mesh. 
C = PCD(g) + PCD(p)
                      2 

Circular Pitch
The distance between adjacent teeth measured along the arc at the pitch circle diameter. 
CP = x MOD 

Circular Tooth Thickness
The width of a tooth measured along the arc at the pitch circle diameter. 
CTT = CP / 2 

Addendum
The height of the tooth above the pitch circle diameter. 
A = MOD 

Dedendum
The depth of the tooth below the pitch circle diameter. 
D = H - A 

Whole Depth
The total depth of the space between adjacent teeth. 
Finer than 1.25 MOD:
H = 2.4 x MOD 
1.25 MOD and coarser:
H = 2.25 x MOD