In Sanskrit and Pali, dhatu means "element" or "property." The Sanskrit word dharma has several meanings. Here it means "manifestation of reality" (Mahayana) or "factors of existence," the transitory conditions that cause phenomena to come into being (Theravada).
In Theravada Buddhism, dharmadhatu (more often spelled "dhammadhatu" by Theravadins) is the fundamental essence of dharma; the nature of things and how they exist.
In Mahayana Buddhism, dharmadhatu is a matrix, totality, or limitless, all-pervading space in which all phenomena arise, abide, and cease.