When two lineages stem from the same branch point, they are sister taxa. A branch with more than two lineages is a polytomy.

What is an example of polytomy?

If the lineages in the phylogenetic tree stand for species, a polytomy shows the simultaneous speciation of three or more species. In particular situations they may be common, for example when a species that has rapidly expanded its range or is highly panmictic undergoes peripatric speciation in different regions.

What is a polytomy in phylogenetic tree?

Usually, a polytomy means that we don't have enough data to figure out how those lineages are related. By not resolving that node, the scientists who produced the phylogeny are telling you not to draw any conclusions — and also to stay tuned: often gathering more data can resolve a polytomy.