It's actually Meta, emulated by an ESC character by the terminal. OSX tip: check "use Option as Meta" in Terminal's preferences, and you can keep Option pressed while you mash ".".
It also has the side effect of "resurrecting" dead keys (which are unrelated to Option), making input of 'ê'and 'ï' impossible.
An alternative is using iTerm2 which has a similar setting, liberally applyable to either Option key.