Linux Mint. How to set the terminal password to be invisible?

Use visudo to remove any mention of 'pwfeedback' in your sudoers file. If on, it's usually under one of the 'Defaults' lines.

You can also add a ! before any instance of pwfeedback in the file to explicitly disable it if you need finer-grain control.

Edit: If you're uncomfortable using vi you can also specify the editor with something like EDITOR=nano sudo -E visudo

LinuxMint added the behavior in /etc/sudoers.d/0pwfeedback.

You could simply do like I did, delete the file as it contains only that adjustment:

sudo rm -rf /etc/sudoers.d/0pwfeedback

The accepted answer of simply removing /etc/sudoers.d/0pwfeedback will work precisely until the next update to mintsystem puts that file back in place.

The way to revert to the old behavior that is recommended by the person responsible for the new one is to override the setting in a different file that gets read later:

echo 'Defaults !pwfeedback'|sudo tee /etc/sudoers.d/9_no_pwfeedback