After reviewing, I've understood two meanings people give to Theta(n):

    1. Theta(n) means that it's O(n), O(n^2), O(...), but 
       Theta(n) is a proper, tighter definition of the upper
       bound.  It doesn't say anything about the lower bound.

    2. Theta(n) means that it's tightly bounded up and down 
       by n. Both Omega(n) and some O(n) are equal, thus its 
       Theta(n).

I think 2 is the proper definition (thus my 2nd edit), but I'm not totally sure. I've let my original comment there with the edit chain in hope discussions would bring up the proper definition. Unfortunately I'm getting downvoted for that, which I find odd.