Why is fish not considered as meat? While reading a thread on cooking, an old question popped into my head: I am an Asian and had no problems with dishes with both meat and “fish.” But some of my elder German friends say that meat and “fish” don't fit.

Why is fish not considered as meat? And what kind of species are considered as "fish"?

Some thoughts about that:

When I was a child, I thought every creature that has muscle tissue has meat. Fish (these animals that swim in the water and have fins like Nemo or your goldfish) included.

A friend of mine calls herself a vegetarian. I thought a vegetarian is someone who doesn't eat meat or more explicit: Someone who avoid dishes that contain parts of something that has a central nervous system or called "animal." But she eats fish. 
Another case: On Good Friday (or Friday in general?!) some Christians in Germany (or anywhere else) eat "vegetarian" food - but including fish.

Then I heard about the biblical story: God hated the humans but liked Noah, Noah built an arch, rescued himself, his family and some animals and God made it rain until everything drowned. The only species that didn't drown... fish. So I thought fish were the holy animal since the Protestants had a fish as a symbol (aside from the cross).

And what about shrimps, sea urchins, sea cucumbers, squids, clams, crustaceans (I avoid to use “shellfish” ;D), …? They are also called “fish.” I don’t think that the bible would refer to these kinds of … fish. The texture of these compared to real fish is completely different.

After some googling, I found some “nutrition scientists” in a forum who claim that fish are no mammals and therefore have no “meat.” But then I would conclude that poultry and reptiles would neither have “meat.” Other claimed “scientists,” say that only animals with red meat would have meat. No, then whale meat, tuna meat, duck meat, and beef would exist but no chicken meat, turkey, pork, ...

A mutual property of real fish, shrimps, sea urchins, sea cucumbers, squids, clams, crustaceans is the ability to stay for some time in the water (sea, lake, river or pool) and the ability to reproduce themselves by laying (?) eggs. What about sea snakes, jellyfishes, lungfish, whales, dolphins, seals, frogs, turtles and other animals which go for some time from water into land and vice versa?
I suspect that this is a question that it's impossible to give a definitive answer for.
In reality it's probably a mixture of religion, culture and confusion.
I reckon that in most cases that it boils down to &quot;Fish isn't a meat because when I was growing up I was told it's not a meat&quot;, or something like that.
In terms of etymology, &quot;meat&quot; originally just meant &quot;food&quot; and as such could be used for food of any kind. This carries over to some extent in modern usage - we sometimes talk about meat of a fruit/vegetable to describe the inside of it, coconut for example.
Personally I use meat to describe animal flesh regardless of the origin, I don't view the meat/fish deviation as being one that is either meaningful nor useful.
RE: fish and vegetarianism,
Fish isn't vegetarian.
Phew, glad we could clear that up.

There's an accepted meaning for the word and it excludes the eating of fish. There are a lot of people who eat fish and are otherwise vegetarian*, which is perfectly fine, but it's a misuse of the word. At the end of the day we all eat what we're comfortable eating, but it can be difficult grouping that many variations. This confuses others and in the end leads to inconvenience to vegetarians.
Oh language...
*Although this probably isn't true. If you're eating fish than it's obviously not on moral grounds so you're less likely to exclude animal products in some of the more obscure places, like may be in wines, cheeses, or indeed any processed goods.