To describe lower bounds we use the big-omega notation f(n)=Ω(g(n)) usually defined by saying for some constant c>0 and all large enough n, f(n)≥c g(n). This has a nice symmetry property, f(n)=O(g(n)) iff g(n)=Ω(f(n)). Unfortunately it does not correspond to how we actually prove lower bounds.
For example consider the following algorithm to solve perfect matching: If the number of vertices is odd then output "No Perfect Matching" otherwise try all possible matchings.
We would like to say the algorithm requires exponential time but in fact you cannot prove a Ω(n2) lower bound using the usual definition of Ω since the algorithm runs in linear time for n odd. We should instead define f(n)=Ω(g(n)) by saying for some constant c>0, f(n)≥ c g(n) for infinitely many n. This gives a nice correspondence between upper and lower bounds: f(n)=Ω(g(n)) iff f(n) is not o(g(n)).
On a related note some researchers like to say f(n)∈O(g(n)) viewing O(g(n)) as a set of functions. This trades off a nice clear unambiguous notation with something ugly for the sake of formality. Yuck.