Strength is a measurement of the fighting prowess of a unit in Civilization V. There are two different types of Strength, which can be modified by factors such as promotions and terrain:
Combat Strength determines how much damage a unit inflicts with melee attacks and how much damage it sustains when defending against attacks of any sort (in which case it can be called "Defense Strength"). Notably, the terms "Strength" and "Combat Strength" are sometimes used interchangeably.
Ranged Strength determines how much damage a unit inflicts with ranged attacks. Attacks that use 
Ranged Strength cannot capture cities, but defenders cannot retaliate against them as they can against melee attacks (unless the attacker is an air unit).
Strength should not be confused with hit points (HP), which determine how much damage a unit can sustain before dying.
See also[]
- Combat (Civ5)
- Strength in other games