Definition:Idempotent Operator

Definition
Let $\HH$ be a Hilbert space.

Let $A \in \map B \HH$ be a bounded linear operator.

Then $A$ is said to be (an) idempotent (operator) $A^2 = A$.

Also known as
Some sources refer to this concept as a projection.

However, another common convention (especially when dealing with Hilbert spaces) is to demand also that it be Hermitian.

In doing this, one arrives at what is called a projection on in this context.

Also see

 * Complementary Idempotent