In compiler theory, a reaching definition for a given instruction is an earlier instruction whose target variable can reach (be assigned to) the given one without an intervening assignment. For example, in the following code:
d1 : y := 3 d2 : x := y
d1 is a reaching definition for d2. In the following, example, however:
d1 : y := 3 d2 : y := 4 d3 : x := y
d1 is no longer a reaching definition for d3, because d2 kills its reach: the value defined in d1 is no longer available and cannot reach d3.