In the context of ATP (adenosine triphosphate) and ADP (adenosine diphosphate), the letter E stands for energy.

Here's why:

* ATP is the molecule that cells use as their primary energy currency. It contains three phosphate groups.

* ADP is formed when one phosphate group is removed from ATP. This process releases energy that the cell can use for various functions.

Therefore, the "E" in the abbreviation represents the energy that is released or stored within the phosphate bonds of these molecules.