Let $X$ and $Q$ be modules. We say that $Q$ is an injective hull or injective envelope of $X$ if $Q$ is both an injective module and an essential extension of $X$.

Equivalently, $Q$ is an injective hull of $X$ if $Q$ is injective, and $X$ is a submodule of $Q$, and if $g:X\to Q^{{\prime}}$ is a monomorphism from $X$ to an injective module $Q^{{\prime}}$, then there exists a monomorphism $h:Q\to Q^{{\prime}}$ such that $h(x)=g(x)$ for all $x\in X$.

Every module $X$ has an injective hull, which is unique up to isomorphism. The injective hull of $X$ is sometimes denoted $E(X)$.