Difference between "fallback" and "fall back"?

In general, grammatically, fallback is a single word, normally a noun. The meanings are what you see on the wikipedia page and notice that in all cases, they're nouns. On the other hand, fall back is two separate words, a verb and an adverb (except in the case of the fall back and forward protocol). So you can fall back, but you can't fallback. You can have fallback, but you can't have fall back.

In the context of data communications, "fallback" is an algorithm used in some protocols. As a verb, "fall back" is used. As a noun, "fallback" is used with no space. So a modem will fall back to a lower speed. The amount of fallback is negotiated between the communicating modems.