Most specifically, ‘affluence’ refers to something flowing to (as opposed to ‘effluence’, which flows from). For some reason, affluence has become associated with material posessions unlike effluence, which is mostly connected to raw sewage (which hopefully flows from and not to). If you’re affluent, you have an increasing supply of material possesions, especially money.In this rather technical view, anyone who has increasing material wealth is affluent–even if their absolute wealth is low–and anyone whose wealth is decreasing is not affluent–even if they’re already rich. Presumably, the reverse is true as well and if you are losing wealth then you’re effluent. Unfortunately, I think the connotations might lead to some misunderstandings.
Anyone who’s had to sit through my rambling about the merits of various savings accounts (Maggie) can tell you that I love to talk affluence. There’s a lot of similarity between the ideals of increasing your wealth and increasing sustainability. If you spend too much money, you’re borrowing against the future. That might work for a little while, but eventually it’ll catch up to you. Sustainability is very similar. We can use lots of resources now, but if we don’t cut back to the level of affluence (increasing resources), we’re just sitting in… well… a big pile of effluence.
Most people now use affluent as a synonym for rich or wealth, which is a shame. You’ll probably be understood, but you lose the (cool) connection to effluence. And, as arduous points out, this definition makes affluence relative (and hence relatively useless, sort of like trying to define people’s religions for them).
There’s a lot of poverty and lack of education and disease around the world and that needs to be fixed. That’s not really connected to sustainability, though. I don’t care how poor you are, you can still work to live sustainably. In some ways, it’s easier to be sustainable when you’re poor because you just can’t afford to consume all the resources that others do.
And, similarly, I don’t care how rich you are, there’s always more you can do. Even if you’re Ed Begley Jr. and ride a bike back to your solar-panel covered house by way of one of your many windmills, you can always help other people realize that living sustainably doesn’t have to mean living poorly.