Plurality. It is the ultimate aim of electing parliaments and MPs. No representatives that haven't got the absolute most support of everyone (all opinions considered).
AV nor FPTP deliver this in every situation, sad as that is given the options we have in this referendum coming up.
But we need to judge the systems on the merits they have, in comparison to each other. A condorcet method, nor a PR-esque method if on the table. We're being asked do we want to stick with what we got or go with AV. To make the move to AV we need to know it's an improvement.
I think it demonstrably is, but some get caught up on this whole plurality lark.
Right now we have FPTP which returns MPs from 2/3rds of the constituencies of our land, somewhere near 400 or more seats, a huge number, who don't necessarily have the most support from their constituency. By using FPTP we are opting for a "simple" system that doesn't give us enough information to tell if local voters can be happy and feel they are properly represented nationally.
This isn't to say those MPs *aren't* the most popular, but with our simple FPTP system we cannot know. We are simply making an assumption that they are because it is logistically easy for us to do so and ensures that winners can be reported first thing on a Friday morning.
AV ensures that at the very least the constituency gets an MP that it doesn't hate...or more accurately ensures that the candidate elected has the endorsement of over 50% of the original turn out, and where that candidate doesn't (as can happen under FPTP in those 400+ seats) at least unlike FPTP we can ensure that they are the least hated candidate.
AV is not a system where one vote is used by every person to go in to a perfect equation or system that ensures the absolutely most popular, the "plurality choice" is elected. It's disingenuous for anyone on the "Yes" side to claim otherwise. Instead we have to think about the break down of the system and what is being asked.
Everyone is given a first choice, and that first set of preferences decides who the most popular MPs are. From that point we know the last placed candidate, the least liked. In the second round we aren't asking everyone to vote again, as in a "run off" system, but to have pre-placed their preferences so that if their candidate is removed to enable their vote to move to someone that is still in the running.
This isn't the way to gain a view of plurality. But the system isn't trying to attain that, it's a series of FPTP elections held sequentially, with a diminishing list of candidates at each stage. Those supporting the weakest candidate are essentially being asked "Look, you can't have that choice, choose from this lot instead".
Come the end what we're doing is collectively judging our candidates. We're coming to a consensus in each round of elimination as to who is the worst of the candidates remaining, but not then (like FPTP) telling those voters that support the worst candidate in that round that their further opinion doesn't matter.
This isn't plurality, but it doesn't mean that it isn't fair; it's just a system that delivers a different way to come to a consensus over the right candidate for the constituency.