What do people mean when they say ELO?

I've noticed a lot of people talking about Elo and I find it a little confusing because there's so many different rating systems, and playing formats.

First, there's OTB (over the board) ratings of FIDE, and USCF. And then there's online ratings of chess.com and Lichess. Playing over the board in a tournament is much different than playing online.

In addition there's a wide variety of time controls people are playing.

Finally, the different chess organizations implement the Elo rating system in different ways.

For these reasons, questions like "My Elo is 1800, how can I improve" means very different things if you're talking about 1800 in OTB tournaments, vs. online blitz, and then USCF will be different than FIDE, and chess.com will be different than Lichess.

For just one concrete example: Magnus Carlsen's FIDE rating in classical is 2835 and blitz is 2887, while his chess.com blitz rating is 3253.

Maybe I'm over thinking it, but I think it would be helpful to specify what time control and chess organization you're talking about when mentioning your rating..

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By the way, the rating system fide first used was created by Arpad Elo, a physics dude. Therefore, many just call their rating an elo even when it's a different formula.

What they probably mean is their FIDE rating, or potentially their domestic federation rating where that federation uses Elo (some use Glicko, or amended Glicko systems, and there are probably some which use other systems altogether).

But generally you have to work it out from context.

It would, as you say, be helpful if people could specify what rating they're actually referring to. A FIDE-registered player who plays on chess.com is likely to have not only two FIDE Elo ratings (classical and blitz) but domestic federation ratings too (which may or may not be Elo) and chess.com ratings, potentially in four different formats of standard chess (daily, rapid, blitz, bullet), which are not Elo but look like they are and are often referred to as such.

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Just ask, it’s what I do. But generally they mean your highest number lol

Elo is the rating but actually over the board