To be more pedantic, it's actually the product of [amplitude] times [complex conjugate of amplitude][0]. Confusingly, this is sometimes called the "square" of a value - usually in contexts where complex-element vectors and matrices are involved, since most[1] real-element formulas calling for matrix transpose (dot products[2] are a special case of this) actually want conjugate transpose[3] but didn't notice because conjugate is identity on reals.

0: zz* = (z)·(z*) = (a+bi)·(a-bi)

1: Possibly all - I have yet to happen across a counterexample.

2: https://en.wikipedia.org/wiki/Dot_product#Algebraic_definiti...

3: https://en.wikipedia.org/wiki/Conjugate_transpose