How to make xterm to send the Alt key to emacs as Meta?

As seen in the Arch Wiki, you need to make Xterm to send the correct escaped key to the program:

If you use the Alt key for keyboard shortcuts, you will need this in your resource file:

XTerm*metaSendsEscape: true

So, just do:

echo 'XTerm*metaSendsEscape: true' >> ~/.Xresources

And restart your Xterm session.

There is an interesting alternative to managing your .Xdefaults / .Xresources files by providing the resource string directly as xterm command line option:

xterm -xrm 'xterm*VT100.metaSendsEscape: true'

This is handy, for example, if you have a custom alias for x-terminal-emulator anyway.

If updating ~/.Xresources doesn't work for you, do as the documentation says and update it in ~/.Xdefaults

http://frequal.com/linux/XTermMetaSendsEscape.html