An ARM-based multicore CPU that uses one or more pairs of high-power and low-power "virtual cores" for greater battery life. Depending on the processing demand, either the big or little core is used. The big.LITTLE architecture was introduced in 2011 along with the first low-power Cortex A7 CPU that was designed to be paired with the high-power A15. The ARM 53-57 pair was announced subsequently.
Why Not BIG.little?
While the obvious upper/lower case designation for this would be BIG.little, somebody thought big.LITTLE would be more ingenious. Go figure. See ARM
and Exynos 5 Octa