Is in-between an actual word?

When I type inbetween, it gets underlined, but in-between does not. However, when I write in word it sometimes underlines in-between in green (for grammar errors) and tells me to replace it with plain old between. Is there a difference between these two words? Or is in-between something people use without realizing it isn't even a real word? It's been driving me nuts so thanks in advance!