A flat round is a financing at the same valuation as the previous priced round, signaling that the company hasn't grown its valuation between rounds. For example, a new Series B at the same post-money as Series A. It often serves as the warning sign before a down round, distinct from an extension round (which uses the previous round's exact price and terms) by being a new round designation with new terms but at unchanged valuation. It is one of the harder-to-classify outcomes in venture financing because the signaling is ambiguous between "patient market timing" and "stagnant performance."
The structural distinctions: