An accent symbol placing a small circle above a character, commonly used to denote special metrics or differential operators in geometry.
Examples
Ring metric notation in differential geometry
\mathring{g}_{ij} \text{ represents the background metric}
Interior point notation in topology
\mathring{A} = \{x \in A : x \text{ is an interior point of } A\}
Ring operator in functional analysis
\mathring{\Delta} \text{ denotes the interior of the Laplacian operator}