meta-begin-environment-regexp

meta-begin-environment-regexp is a variable defined in `meta-mode.el'.
Its value is

"\\(begin\\(char\\|fig\\|gr\\(aph\\|oup\\)\\|logochar\\)\\|def\\|for\\(\\|ever\\|suffixes\\)\\|if\\|mode_def\\|primarydef\\|secondarydef\\|tertiarydef\\|vardef\\)"

Documentation:
Regexp matching the beginning of environments to be indented.

You can customize this variable.