An armor module is the design feature of a C-evo unit that gives it defensive strength.
The basic value/strength of an armor module depends on the knowledge of relevant technologies and is proportionally reduced if the unit's health is below 100%. For units in a city, defensive strength is halved for naval units and zero for air units. Factors that may increase strength come from buildings and terrain.
Every new unit design must incorporate at least one armor module. Players can add more, up to specified limits, depending on the weight limit and (again) the knowledge of relevant technologies.
Ground units are allowed a total of three armor modules after the discovery of Steel but only two before that. The exception is for an Overweight unit, a unit that is restricted in movement but is allowed one more weapons unit and one more armor unit than the standard maximum.
Naval units are allowed a total of four armor modules after the discovery of Steel but only three before that.
Air units have no specified maxima.