As far as I know, when you use key as a separate word like in your example, it's functioning as an adjective that means important or crucial. For example:
The key feature of this new program is that it can now log in users automatically.
That the program can now log in users automatically is an important characteristic of its new version that sets it apart from all previous versions.
America is about freedom, democracy and the pursuit of happiness. The key word here is freedom because without it you can't have the other two.
This says that the most important word out of those three is freedom.
On the other hand, the term keyword is a word that has a well defined meaning which is quite different from when key word is used as two separate words. For example, in the context of search engines or websites, it typically means search term; that is, the thing you're searching for.
That's the way I distinguish these two and it makes sense to me. But I know what you might be thinking: if you look up keyword in a dictionary, it will say, "A word or concept of great significance" as the first definition. That just means that key word can in fact be spelled as one word (but I do think there is a slight different in meaning between them though). You're free to use either one, but I would personally opt for the way I just defined them.