In phylogenetics, basal members of a group diverged earlier than a subgroup of others (or vice versa). It is often used in opposition to the word derived. The term means nothing on its own and can be interpreted only in the context of the group as a whole.

For instance:

The palaeodicots are basal flowering plants, diverging before the main split between the monocots and eudicots.

Orangutans are the most basal of the great apes, having diverged from the line earliest.

Many biologists (especially those that use cladistics) now prefer to use "basal" instead of the word "primitive," which may imply inferiority. On the contrary, if a basal member of a group exists alongside derived members, it has had exactly the same amout of time to evolve and is usually still well-suited to its environment.

Phylogenetics