The notation θ(n) is the formal way to express both the lower bound and the upper bound of an algorithm's running time.
If, $$c1.g(n) <= f(n) <= c2.g(n)$$ for $$c1,c2 > 0 , n >= n0, n0>=1$$ then,
$$ f(n) = Theta(g(n))
$$
