Theta, Oh and Omega Notations

n: the size of input.
Theta of n: indicate that a running time is bounded from above by some linear function of n, and from below by some (possibly different) linear function of n.
Big-oh of n: indicates that a running time is never worse than a constant times some function of n.
Big-omega of n: indicates that a running time is never better than a constant times some function of n.