(
BIT BLock 
Transfer)  In computer graphics, a hardware feature that moves a rectangular block of bits from main memory into display memory.  It speeds the display of moving objects (animation, scrolling) on screen.
A hardware bitblt provides fastest speed, but bitblts are also implemented in software, even in non-graphics systems.  For example, text scrolls faster when it is copied as a contiguous block (bitblt) to the next part of the window, rather than processing every character on every line.  See 
stretch blt.