Step 1: ATP stands for adenosine triphosphate.
Step 2: Structurally, ATP is composed of a nitrogenous base (adenine), a sugar (ribose), and three phosphate groups.
Step 3: This structure classifies ATP as a nucleotide.
Step 4: ATP functions as the energy currency of the cell but is not an enzyme, vitamin, or nucleic acid.
Hence, ATP is a nucleotide.