Q: What is the opposite of within?
Don't tell me outside (which is opposite of inside).
Any one word that can be used?
A: You might not want me to tell you this, but the opposite of within is "outside of" (in English). Inside basically means the same as within.
Interestingly, we Scots (Scottish English is the variety of English spoken here) have come up with a different word which the English (of England) don't use. The word is "outwith" and it is the opposite of "within". "Without" actually means "lacking" or "minus".
Don't use "outwith" outwith Scotland or no one will know what you're talking about!