Short for Double Data Rate-Synchronous DRAM, a type of SDRAM that supports data transfers on both edges of each clock cycle (the rising and falling edges), effectively doubling the memory chip’s data throughput. DDR-SDRAM also consumes less power, which makes it well-suited to notebook computers. DDR-SDRAM is also called SDRAM II. and DDRAM.
Also see the Guide to System Memory page in the Quick Reference section of Webopedia.