From encyclopedia Britannica:
in military science, crew-served big guns, howitzers, or mortars having a calibre greater than that of small arms, or infantry weapons. Rocket launchers are also commonly categorized as artillery, since rockets perform much the same function as artillery projectiles, but the term artillery is more properly limited to large gun-type weapons using an exploding propellant…

There is room for argument as the definition varies quite a bit from source to source. Originally the mortar was a large weapon that was hard to relocate. For example the units used during the war between the states. Those mortars used a propellant and shell. Modern mortars like the 60 mm and 81 mm are essentially rocket launchers. They are closer related to the MLRS than to a howitzer. You could make a point that there are two types of artillery. The gun style and the rocket based. It is my opinion that true artillery is gun based.

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