Identität
identity in maths usually means an equation that is true for all allowed values. Example:
x + x = 2x
for x=3 we get "3+3 = 2*3" which is true.
for x=4 we get "4+4 = 2*4" which is true.
for x=0,5 we get "0,5+0,5 = 2*0,5" which is true.
for x=-3 we get "-3 + (-3) = 2*(-3)" which is true.
In fact, any number put in place of x makes the equation become true. That is what an identity means: Whatever you put for x the equation will always become true.