Ranked Choice Voting (RCV) technically refers to a family of voting systems that use voter's rankings to determine the winner. Instant Runoff Voting is the most common RCV system in use today. Because STAR Voting actually uses the ranking derived from scores to make the final runoff decision, it is arguably an RCV system, but it is not at all the same as Instant Runoff Voting. STAR is similar to IRV in that voters can show relative preferences between candidates, but there are important differences. For example:
With STAR Voting you can show that you like two candidates equally; with IRV you would have to rank one over the other.
In STAR Voting everyone's full ballot is counted; in IRV your down ballot rankings may never be counted, depending on the order of elimination.
In STAR Voting there is only one automatic or "instant" runoff; with Instant Runoff Voting there can potentially be many instant runoffs.