(Or cache block) The smallest unit of memory than can be transferred between the main memory and the cache.

Rather than reading a single word or byte from main memory at a time, each cache entry is usually holds a certain number of words, known as a "cache line" or "cache block" and a whole line is read and cached at once. This takes advantage of the principle of locality of reference: if one location is read then nearby locations (particularly following locations) are likely to be read soon afterwards. It can also take advantage of page-mode DRAM which allows faster access to consecutive locations.