Vertical synchronization
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Vertical synchronization (v-sync, v-synch, vbl-sync, vbl-synch) refers generally to the synchronization of frame changes with the vertical blanking interval. Since CRTs were nearly the only common video display technology prior to the widespread adoption of LCDs, the frame buffers in computer graphics hardware are designed to match the CRT characteristic of drawing images from the top down a line at a time by replacing the data of the previous frame in the buffer with that of the next frame in a similar fashion. If the frame buffer is updated with a new image while the image is being transmitted to the display, the frame buffer gives it the current mishmash of both frames, producing a page tearing artifact partway down the image.
Vertical synchronization eliminates this by timing frame buffer fills to coincide with the vertical blanking interval, thus ensuring that only whole frames are seen on-screen.
Sophisticated software such as computer games and CAD packages often allow vertical synchronization as an option, because it delays the image update until the vertical blanking interval. This produces a small penalty in performance, which can be enough to reduce a program's frame rate below the refresh rate of the monitor on computationally strained systems.
VSYNC is also the name of the signal indicating this frame change in analog RGB component video.
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