A Hebrew term literally meaning "mixture." It is often used in the context of one of several legal constructions which Jews use to finesse some restrictions.
The best-known example is the eruv chatzerot, literally "mixture of courtyards." As described below, it uses some really fascinating bits of legal fiction.
On Sabbath, we are forbidden to carry things between one "domain" and another. Domains are things like a person's house and yard vs. another person's property, or the public domain, etc. Properly speaking, there are four kinds of domains: private, public, exempt (a small area, less than about a yard square, under some circumstances can be carried into and out of from any domain; we won't be dealing with this kind), and "karmelit," which I can't translate well. Let's say "common domain," OK? The last is not really public domain, since the "public" in question isn't big enough, but it's more than private domain. So maybe a shared courtyard among several houses. You can't carry between private domains, between public domains and private domains, or within the public domain at all. Rabbinically, the common domain is treated like a public domain, so you can't carry to or from or in there either. The qualifications for being a true public domain are pretty stringent, involving things like 600,000 people (or maybe actually 600,000 Jews) passing through it on a daily basis, so in actuality just about everything we think of as public areas are really "common" domains (which is significant, because the prohibition there is only Rabbinic, and so we can finesse it some).
The trick is to turn the common domain (and all the private ones, for that matter) into one big (commonly-held) private domain. How do you do this? The same way you do for your own backyard (which isn't part of your private domain either until you do this): build a wall around it. When you enclose an area in a wall, it becomes (or can become) a private area. Ah, but walling a city is a pretty big proposition! And yet, there's one around many cities world-wide, even my own town in New Jersey. But nobody seems to notice. The trick is that the wall is made mostly out of legal fiction and very little actual matter. This is neat to watch.
First of all, how do you make a wall you can still walk through to get in and out of the town? Easy: you make a door. A door is still considered part of the wall, right? So what if I had a wall made entirely of doorways? That would be a wall you could walk through anyplace, except at the small doorposts of course. That's the main insight: we'll make the wall of only doorways. But that's still a bit much for a large town, so let's try to break it down some. What's a doorway anyway? Well, it's two doorposts and a lintel. If it has that shape, it's a doorway, otherwise, well, it isn't. So we could use, say, telephone poles and wire: the poles form the doorposts and the wire the lintels. No, really, that could work! In fact, it's almost what's used. But it isn't quite right, since telephone wires don't pass on top of the poles, but are held off to the side, and that's not considered the proper shape of a doorway, with the doorposts supporting the lintel. We could just run another small pole, even a wire, really, down the side of the telephone poll which was directly under the cable on top; that would work and is actually done. Really! But that's not always feasible either. So we use some various legal concepts that let us make it even easier. Things like "something close enough to the ground (about a foot/30cm or so) is considered on the ground." So now the bottom end of my "doorpost" doesn't have to be actually on the ground. And "something tall enough (about a yard/meter or so) is considered to go up and up indefinitely." So I don't need the doorpost to reach all the way to the "lintel"; just be a few feet tall directly underneath it.
So between one thing and another, using telephone wires and fences and occasionally stringing their own wires, synagogues all over the place have managed to erect these "walls" around cities all over the place--maybe even your own hometown! There's a bit more to it; they need permission from the local government (not just to mess with the utility poles but also just to make the "mixture" work: it has to be with the consent of the town), and there's a bit of other ceremony (involving buying a certain amount of food and storing it where it can be used by anyone at need), but basically that's how it works. Amazing what a few legal concept can build, and cheaper than a contractor!
There are other kinds of eruvim. The other one most commonly known about is the eruv tavshilin or "mixture of cooked food." This one deals with the case of a 2-day holiday (when the second day is a Sabbath) or a holiday right before Sabbath. One is allowed to cook on a holiday (but not on Sabbath), but is Rabbinically forbidden from cooking food for another day. So you can cook for today, but not tomorrow. This is a problem when the next day is a Sabbath, when you can't cook. When can you cook for Sabbath? Well, you can leave over stuff from today to tomorrow, and even cook a lot on purpose even though you know you're only going to eat a little today and the rest will be left over. So that's the basis of the eruv. You set aside some food before the holiday (usually a boiled egg and a piece of matzo) and so start your preparations for Sabbath from way before, and in effect declare that all the food you'll be cooking over the holiday is all "mixed together" and in one batch with the stuff you started with. Since that's started before the holiday and left over, you can cook anything you like for Sabbath and it's still in the same "batch" as stuff for the holiday, so you can "leave it over."
The only other kind of eruv I know offhand is the eruv t'chumin, which is little-used, and enables one to finesse the restriction on travelling more than 2000 cubits outside of town on Sabbath. It involves leaving food (before Sabbath) at the limit of your 2000 cubits in one direction, and that establishes your "place" there. So then you can go another 2000 cubits beyond it on Sabbath (but you lose your journey-distance in the other direction).