Ultrametric tree
From BioPerl
A tree with equal root-to-tip path lengths for all lineages, for example one built with the assumption of a molecular clock.
Itai Yanai, Molecular Genetics Department, Weizmann Institute breaks it down like this:
"An additive tree is one where along with the topology of the tree edge lengths are also specified corresponding to distances among the organisms. An ultrametric tree is a special kind of a rooted additive tree where the terminal nodes are all equally distant from the root."
