Nearest Neighbor Interchange. This is a heuristic algorithm for searching through treespace. It proceeds by juxtaposing the positions of neighbors on a phylogenetic tree. If the resulting tree is better, then it is retained. This algorithm is quite a gentle perturbation of the tree and is inferior to either SPR or TBR in terms of completeness of the searsh. On average it will be quicker than SPR or TBR.