A relational symbol denoting the restriction of a function to a smaller domain in analysis and set theory.
Examples
Function restriction to a subset
f\restriction_A: A \to \mathbb{R}
Restriction of a continuous function to an interval
g\restriction_{[0,1]}: [0,1] \to \mathbb{R}
Restriction of a map between topological spaces
\phi\restriction_U: U \to V \text{ is a homeomorphism}