- any of the Thallophyta, a plant division in some older classification schemes, comprising algae, fungi, and lichens.
- obsolete any organism of the former division Thallophyta, lacking true stems, leaves, and roots: includes the algae, fungi, lichens, and bacteria, all now regarded as separate phyla
- Any of a former group of plantlike organisms showing no differentiation into stem, root, or leaf. Thallophytes were regarded as constituting a major division of the plant kingdom and included the algae, fungi, and lichens. No longer in scientific use.