Zero to the power of zero
It is determined by definition that 00 ≝ 1, as you cannot calculate it's value.
Explanation
Exponentiation means "multiply the factor 1 so often with the base as the exponent of this indicates", so
Written in more detail you get
a3 = 1 · a · a · a
a2 = 1 · a · a
a1 = 1 · a
a0 = 1
Zero as exponent tells that factor 1 must not be multiplied with the base, so only 1 remains. This notation produces 00 ≝ 1 and that is good.