Final answer:
A lineage in a phylogenetic tree that diverges early from the rest and remains unbranched is known as a basal taxon. This term reflects its position in the tree, not its level of complexity or how evolved the organisms within the taxon may be.
Step-by-step explanation:
In a phylogenetic tree, a lineage that diverges from all other members of its group early in the evolutionary history of the group is described as a basal taxon. A phylogenetic tree's root indicates that an ancestral lineage gave rise to all organisms on the tree, and a branch point represents where a single lineage evolved into a distinct new one. When two lineages stem from the same branch point, they are known as sister taxa. A branch with more than two lineages is referred to as a polytomy, illustrating uncertainties in evolutionary relationships. It is essential to recognize that a basal taxon is not necessarily more primitive or less evolved but simply represents an early and separate evolutionary path within the group.