Memory addressing in which each byte is referenced by a base number (the segment) plus an offset. An x86-based PC running in 16-bit mode (Real Mode and Virtual 8086 Mode) uses 64KB segments, and a segment register always points to the base of the segment that is currently being addressed. Although the PC 32-bit mode is considered a flat address space, it too uses segments. However, the segments are 4GB, huge by comparison to 64KB segments, and more than enough to handle all the memory most users require. See
Real Mode and
Virtual 8086 Mode.