The small-step operational semantics of a graph is an operational semantics given by a set of reduction rules on an AST or other intermediate representation after syntactic analysis. Each reduction defines a small fixed-size step from an abstract machine state to the next state.
Each reduction rule either defines a single reduction, or adapts a reduction by embedding the redex in a larger expression.
For example, see page 3 of this https://groups.seas.harvard.edu/courses/cs152/2021sp/lectures/lec02-smallstep.pdf
This contrasts big-step operational semantics