Pi
In Adenosine triphosphate and friends, energy is stored in the bonds between these. Imagine walking a dog. That's Adenosine monophosphate. Now, imagine attaching another leash to the collar of the fist dog, and another dog to the other end of that leash. That's Adenosine diphosphate. Add another leash and dog, this time attached to the second dog. That's Adenosine triphosphate. Fun.