Definition of RLL

RLL: Acronym for the right lower lobe (of the lung).

The right lung has three lobes: the right upper lobe (RUL), the right middle lobe (RML), and the right lower lobe (RLL).

The left lung has but two lobes: the left upper lobe (LUL) and the left lower lobe (LLL).

The reason why the right lung has three lobes whereas the left lung has only two is not known but a convenient (albeit teleological) way to think about it is that the lack of a lobe on the left leaves room for the heart on that side.