Keyboard shortcuts in green and mnemonics in yellow.

The best keyboard related definition I’ve found for mnemonic it was on Sun Java glossary:

mnemonic – An underlined alphanumeric character, typically in a menu title, menu item, or the text of a button or component. A mnemonic shows the user which key to press (in conjunction with the Alt key) to activate a command or navigate to a component.

If you had Win32 experience you may call them “menu accelerators”. They define the letter used for triggering them by using Ampersand (&) inside the menu text. I would not recommend the usage of “accelerator” term because looking over the net I’ve found that in many cases it does refer to keyboard shortcuts.

If you are wondering why it’s important to clearly distinguish between them: it’s because the mnemonics are changing when the UI language is localized but the keyboard shortcuts are not supposed to be localized.
.