Sprite strength is how many shots are required before a sprite 'dies' Most sprites require only a single shot to change what they do. This cannot be easily altered and these sprites are considered to have no strength. Most sprites are like this. Some however have a separate variable that is tracked. When this is zero, the sprite 'dies'. This is sprite strength. Sprite strength can only be found by default in Keen 1, Keen 2, Keen 3 and Keen 6, though other games can have it patched in.
Following is an example from Keen 1, the Vorticon. The vorticons in Keen 1 can have two possible strength values, 4 or 100, depending on what level they are in. All Vorticons in level 16 share the increased strength..
#Strength: %patch $180D [$0003W] #Strength of regular Vorticon -1 %patch $1813 $10 #Level on which Vorticon commander appears %patch $1819 [$0068W] # Strength of Vorticon commander -1
Most strength patches follow this example; setting the strength when the sprite is spawned, decreasing it by one each time the sprite is shot. Closely related is Patch:Sprite collision. Individual strength patches can be found on the pages of various sprites while more generic collision\strength patches are on the sprite collision page.