What is the default META key for Irssi (IRC) on macOS? What is the default Meta key for Irssi (IRC) in Terminal? 

I noticed that I can use the ⌥ Option key as Meta key if enabled in Terminal settings, but this prevents me from using it to type special characters.

If there is no default Meta key, is it possible to define one different from ⌥ Option?
According to Irssi documentation, it's Alt, which I believe the author meant ⌥ Option to be on macOS, as this is how ⌥ Option is usually used on macOS compared to Windows.

However, on macOS Mojave, it is actually Esc.