The compound formed by the combination of ADP (adenosine diphosphate) and an additional phosphate group is called ATP (adenosine triphosphate). ADP is a nucleotide that consists of adenosine and two phosphate groups, whereas ATP is the phosphorylated form of ADP, with three phosphate groups attached. ATP is commonly referred to as the "energy currency" of the cell, as it plays a crucial role in cellular energy transfer and storage.