1. Any large concentration of algae drawn up from the sea and collecting on the surface of a body of water, though it may not be exactly red in color and is not dependent on or related to the tides. An unusual number and intensity of storms in this region has contributed to a near-constant presence of red tide, which is frustrating beachgoers and threatening the fishing industry. The sharp uptick of respiratory problems in the seaside town has been blamed on the red tide that has been present ever since the hurricane last month.

2. slang A menstrual period. Potentially offensive if used by anyone other than the one menstruating. Sorry for being so snappish with you, I'm just having my red tide at the moment. A: "I bet she's just down with the red tide." B: "Jeff! Don't say something so presumptuous and crass!"