What is buttermilk?

I've seen 2 things that you call buttermilk. One is the skim milk that's left after whipping heavy cream until it becomes butter, and the second is a fermented milk that's similar to a thinner yogurt. I've seen a lot of recipes of american fried chickens, saying to brine the chickens in buttermilk for the acidity to break down the protein in the chicken. It is clear that they were referring to the second thing, which is fermented, because this is the sour one, The other one, which is a skim milk that remains after making butter is not acidic at all, it is almost water mixed with a spoon of milk. Why these things that don't have anything in common are called the same.