A columnar dataset in ROOT consisting of a list of independent columns, or branches. The branches typically contain variables representing fundamental types, simple classes/structures or list of variables. In ROOT, ntuples are represented with the TTree class (a simpler TNtuple class is provided for ntuples restricted to a list of float variables).
The average number of proton-proton collisions in an LHC bunch crossing. See NuMuPileUp.