Infantry support weapon. A type of cannon that really isn't a cannon - it fires in higher arcs (higher than 45 degrees) and is a muzzle-loading one.
Mortars are lighter than cannons to move around - a small mortar can be handled by just a few men, the smallest, toy sized ones, with one man - and are generally faster to make ready for firing.
(I know next to nothing about mortars - I served in a mortar company. Why civilians always know more about weapons than soldiers?)
Mortar is also the name of the program I created that generates NoCeM reports. It was originally written in Perl, but I later made an Emacs LISP version of it (just for convinience).