Acronym for Run Length Limited encoding.

RLL encoding is a method for storing information on a computer hard disk drives and floppy disk drives. This method of storage is carried out by hardware in the disk controller and is less prevalent than the modified frequency modulation (MFM) encoding scheme. It was invented by IBM.

<confusing explanation>
Groups of bits are mapped to specific patterns of flux. The density of flux transitions is limited by the spatial resolution of the disk and frequency response of the head and electronics. However, transitions must be close enough to allow reliable clock recovery. RLL implementations vary according to the minimum and maximum allowed numbers of transition cells between transitions. For example, the most common variant today, RLL 1,7, can have a transition in every other cell and must have at least one transition every seven cells. The exact mapping from bits to transitions is essentially arbitrary.
</confusing explanation>

Today, virtually every drive on the market uses some form of RLL.

See http://www.pcguide.com/ref/hdd/geom/dataRLL-c.html for more information.