The Free On-line Dictionary of Computing (30 December 2018):

operational semantics

    A set of rules specifying how the state of an actual
   or hypothetical computer changes while executing a program.
   The overall state is typically divided into a number of
   components, e.g. stack, heap, registers etc.  Each rule
   specifies certain preconditions on the contents of some
   components and their new contents after the application of the
   rule.

   It is similar in spirit to the notion of a Turing machine,
   in which actions are precisely described in a mathematical
   way.

   Compuare axiomatic semantics, denotational semantics.

   (1996-08-21)