You said it yourself, any cryptocurrency other than Bitcoin is referred to as an altcoin. Meme coins are not different than any other coin or token created, the only difference is that usually projects create their coins and tokens for some utility. Meme coins are created usually for fun, but in essence, they are altcoins by nature.
The reason why meme coins are so much in trend these days is that retail investors, especially the new ones, are too much into them because they believe meme coins are their only way to gain wealth very quickly, so instead of investing in Bitcoin or proper altcoins, they go for meme coins, and since they are told that mostly new meme coins can grow multiple folds, a lot of meme coins are gaining a lot of investments these days, but only some of them manage to gain value and most people lose money.
Meme coins are like fancy-looking dolls that lure kids towards themselves, and kids, in this instance, are newbie investors.