What does this mean?

What does .md mean here?

HTML

<div class="divider lg"></div>

CSS

.lg {
  height: 10px;
}

.lg, .md {
  background-color: black;
  border: 0;
}

Class names don’t have any special meaning, any class name can be a bunch of random characters.

But, seeing this

I guess the first is short for “large” and the second for “medium”