Dhātu: 'elements', are the ultimate constituents of a whole.
I The 4 physical elements dhātu or mahā-bhūta popularly called earth, water, fire and wind, are to be understood as the primary qualities of matter. They are named in Pāli: pathavī-dhātu, āpo-dhātu, tejo-dhātu, and vāyo-dhātu In Vis.M XI, 2 the four elements are defined thus:,Whatever is characterized by hardness thaddha-lakkkhana is the earth or solid-element; by cohesion ābandhana or fluidity, the water-element; by heating paripācana the fire or heat-element; by strengthening or supporting vitthambhana the wind or motion-element. All four are present in every material object, though in varying degrees of strength. If, for instance, the earth element predominates, the material object is called 'solid', etc.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.