There are several definitions for dhatu in ayurveda. Few of them are given below.
"Shareera dharanathdhatawaha" - The tissues which nourish the body and support it are called as dhatus.
"Poshanadharanayoho" - Those which nourish the successive tissue and support it.
"Dadhati datteya vaa shareera manaha praanaan" - these tissues nourish and support the body, mind and life Dhatus are "Shakti yukta dravyas". i.e they supply nutrients to other vital tissues and keep the body healthy. They nourish and support the successive tissues.