Many of us, along with some textbooks, fall back on the "two consecutive quarters of negative GDP growth" definition. We should all be directing our students to the indicators used by the National Bureau of Economic Research (NBER) definition.
Here is an interesting post from Calculated Risk (HT to Econlog) that speaks to the NBER definition. It also provides a sobering view of where we are in the current cycle. While we can definitely see we're past the trough, we still have a considerable amount of ground to cover to get back to the previous peak.