Meat has few mysteries. It is basically the muscles that animals use to move.
In the kitchen meat can mean a few different things. It can be the general term used for all direct dead animal products that we eat. Like the stuff we eat from a fish is the meat, not the fish-bones and fins but the muscles. Liver (ex from geese or pigs) is not muscle but usually clasified as meat (as are kidneys and lungs and ...).
Milk and butter and cheese are animal products too but they are not classified as meat because they don't come direclty from dead animals.
Very broad.
Eggs could be classified as meat in this way. Most eggs these days are not fertilsed (unlike the old times you are very unlikely to commit an abortion when you crack and egg).
More restricted the term meat is applied to things sold by the butcher. Most of that is the remains of dead mamals. Most of it is the muscles cut up and partitioned in steaks and roasts. Sometimes, as in cutlet or spare-ribs, with some bone or, as in bacon, with skin still attached.
Of course butches sell lungs and kidneys and liver as well so that gets included in the meat as well.
Normally butchers don't sell fish (NEVER!!!!!) nor fowl (heu ... usually not...). The muscles of both are (biologically and medically classed as meat and the kitchen follows. Well if you get a 'drumstick' (thigh of a chicken) you usually don't eat the bone, carthrilage, fat or skin but just the meat. And when you get a piece of kooked bass you don'e eat the skin nor the fish bone but only the ... meat.
Meat from fish and fowl usually is not 'red' (even salmon is not 'blood-red') but rather pale so they don't belong to the 'red meat' category. Of course there are exceptions (duck, tuna, domestic-pigs) but we can gloss over them.
Horror.
A few thousand years ago, if you wanted meat you had to go hunt it down and kill it yourself.
Want a pounded steak? (-5000y)
OK, take your spear and hunt some odd buffalo (typically an old or sic one). After stalking, running, climbing, falling and fasting for several days you finally get it down. You are luckily living in a cold ice age so it doesn't matter that you take twomore days to tow it back to your grotto. Your tropical friend of course arrives with a half rotted and fly infested carcas at his family camp-fire. Poor sod.
OK, you arrive home, that's the point. Now you take a half blunt stone to saw trough the hide and tear trough the sinews. Once the hide is off (your wives are now stretching, scraping and/or chewing it) and the sinews are roughly cut off (your starved four-fith wild dogs are figting over them now) the meat is easily torn of the bones. Of course when you have schlepped the dead animal around for a couple of days the muscles are all hard and stiff (and filled with clogged blood) so you throw it on a rounded stone and beat it with a firm stick till it gets supple again. Once it's supple you throw it at the family member it's intended for or, if it's yours, just start chewing. Alternatively you can throw it for a few seconds on the hot coals of your fire.
Want a pounded steak? (+2y)
Click three buttons on your browser and half an hour later someone brings you a 'custom made' pounded steak that you can put on your electric barbecue.
Ok. Ok. I exagerate a bit (not that much actually) but the idea is there. The main difference between the modern, electric man and his hunting forefather is that the ancient man knows exactly what he gets and his modern kin does not. And neither would recognise the other's dinner as being the same.
Pounded steak is one thing, still more or less recognisable. Minced meat is another, and further still is the prepared stuff like saucages, hamburges and pastes.
I like to stand closer to the fur-clad spear-weilding ancestor. I like to know what I eat. But I happily skip the hunting and dragging part of the story.
I don't do it always but sometimes I buy a steak and chop it up myself in stead of buying a packet of minced meat. It's a lot more work but at least I know what is mixed in. If it were practicable I would, once in a wile, do the slaughtering and butchering just as well.
Bloody meat, just don't cut your fingers along.