I want to create a poster titled "An Incomplex Introduction to Complexity-based Cryptography." As you see, it contrasts the words incomplex and complexity. (Words like simple or easy do not provide this.) I looked up the word incomplex, and found it in a few dictionaries:
However, most dictionaries do not include this word. Is it a legitimate word at all? Can I use it?
Best Answer
It's definitely not a standard English term, and as such will likely sound wrong to most people, even if it is shown somehow to be a legitimate term.
I understand your interest in using it to contrast against "complexity" but I think you would find that "simple" would work just as well, as simple is quite commonly used as the opposite of complex. And you'd get far fewer people wondering if it were a real word.
If I were trying to write the title, I'd probably go with, "A Simplicity-based Introduction to Complexity-based Cryptography." Snappy, and you have full part-of-speech agreement between your contrasting phrases--which makes it crystal clear that you're going for a fun contrast.