Overdraw
From polycount
Overdraw means a screen pixel is being drawn more than once.
Overdraw can increase the fill rate which can slow down the frame rate. Re-rendering each pixel more than once is usually a waste of processing time. Overdraw should be avoided whenever possible.
Overdraw is usually caused by multiple triangles being drawn over each other, for example with particle effects or tree foliage. It is required if triangles are partially transparent, because the surfaces must be mixed together to create the final screen pixel.
