Bit blit (also written BITBLT, BIT BLT, BitBLT, Bit BLT, Bit Blt etc., which stands for bit block transfer) is a computer graphics operation in which several bitmaps are combined into one using a boolean function.The operation involves at least two bitmaps, one source and destination, possibly a third that is often called the "mask" and sometimes a fourth used to create a stencil.