vhdl-forbidden-words

vhdl-forbidden-words is a variable defined in `vhdl-mode.el'.
Its value is

nil

Documentation:
List of forbidden words to be highlighted.
If option `vhdl-highlight-forbidden-words' is non-nil, these reserved
words are highlighted in a warning color to indicate not to use them.

NOTE: Activate the new setting in a VHDL buffer by re-fontifying it (menu
entry "Fontify Buffer").

You can customize this variable.