Effectiveness
A measure of whether the objectives of a practice, service, or activity have been achieved. It is about "doing the right thing."
Impact
A system can be highly efficient (low cost, fast) but completely ineffective (doesn't solve the problem). Focusing on effectiveness ensures alignment with business goals.
Weinto take
Effectiveness is measured by Outcomes, not Outputs. We don't care how many lines of code you wrote (Output) or how many hours you worked; we care if the user's problem was solved (Outcome). We audit our work against the definition of "Done," which includes value realization.