Wiktionary
subthalamus
n. (context anatomy English) Part of the diencephalon beneath the thalamus
WordNet
subthalamus
n. the ventral part of the thalamus
Wikipedia
Subthalamus
The subthalamus or prethalamus is a part of the diencephalon. Its most prominent structure is the subthalamic nucleus. The subthalamus connects to the globus pallidus, a basal nucleus of the telencephalon.