The Stacks project

Definition 6.7.4. Let $X$ be a topological space. Let $A$ be a set. The constant sheaf with value $A$ denoted $\underline{A}$, or $\underline{A}_ X$ is the sheaf that assigns to an open $U \subset X$ the set of all locally constant maps $U \to A$ with restriction mappings given by restrictions of functions.