The story
So called because it is made up of individual "leaves" of a chemical element, usually tungsten, that can move independently in and out of the path of a particle beam in order to block it.
Etymology from Wiktionary, CC BY-SA 4.0.
multileaf collimator
Today it means: A collimator used in a linear accelerator to provide conformal shaping of beams for radiotherapy. Abbreviation: MLC. Full definition →