Expulso is a Spell that causes an object to explode. The force of the explosion may depend on the intent of the caster.
Uses
Antonin Dolohov used this spell in a café on Tottenham Court Road while in pursuit of Harry Potter (though Hermione Granger and Ron Weasley were accompanying him, the Death Eaters did not know this at the time) in August of 1997. It struck the table that Harry was standing behind, causing an explosion that slammed him into a wall with great force.[1]
It is possible this may have been the spell that caused the explosion (which may have been caused by Death Eater Augustus Rookwood), which killed Fred Weasley during the Battle of Hogwarts.
Etymology
Latin expulso meaning to "drive out, expel, force out, banish.