A piece of artillery firing at elevations generally between 45' and 90' from the horizontal.
Calibres range from approximately 70mm upwards to over 400mm in extreme cases and tend to fire a low velocity, thin cased shell designed to deliver effect from its payload - explosives by default, but frequently other agents. The smaller designs are typically man-portable and muzzle loading, larger weapons may be breech loading and transported by rail if at all.
Whatever the size of the weapon, the thin case tends to give a mortar bomb a proportionately high explosive payload for its calibre.
Historically the mortar was regarded as a siege weapon due to its ability to drop shells over walls and into trenches, and it was in this capacity that it came into its own during WW1 when what was previously the unquestioned province of artillerymen was lightened and moved closer to the front line so that it could support the infantry more effectively. The light mortar was the ultimate expression of this trend.
Since WW2 the mortar - generally in a 70-85mm calibre has been normally been attached to infantry units at battalion level and represents the infantryman's artillery of first resort.