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 following are example usages of the term basal:

• 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 more time to evolve and is often better-suited to its environment than modern derivatives.

Alternatively, in botany, the term basal can mean "at the base of"; for example, the wild orchid genus Piperia have leaves that are entirely basally situated on stem. The term can also be applied to the columnar epithelial cells which are found in the human body. In this case the basal surface is opposite the Apical surface of the cell.