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.