\begin{definition}[Definition:Partition of Unity (Hilbert Space)]
Let $H$ be a Hilbert space.
A '''partition of unity''' or '''partition of identity''' on $H$ is a family $\family {P_i}_{i \mathop \in I}$ of projections, subject to:
:If $i \ne j$, then $P_i P_j = P_j P_i = 0$
:$\vee \set {\Img {P_i}: i \in I} = H$, where $\vee$ signifies closed linear span
One may encounter the notations $1 = \sum_i P_i$ and $1 = \bigoplus_i P_i$.
Here, $1$ signifies the identity operator on $H$.
\end{definition}