Spell Name Origins

Accio (summoning charm) - Latin for "to summon."
Avada Kedavra (killing curse) - Aramaic phrase that means "may it be destroyed."
Avis (spell that Ollivander used to make birds fly out of Krum's wand) - Latin for "bird."
Cunjunctivitus Curse (spell that Krum used to "do something" to the eyes of the Chinese Fireball during the first task of the TWT) - Conjunctivitis is the scientific name for pink-eye -- the illness that children often get that makes eyelids crust together.
Cruciatus Curse - "Crucio" is Latin for "to torture."
Deletrius (banishing spell) - Latin for "to erase."
Densuageo (spell that Malfoy used to make Hermione's front teeth grow huge) - "Dens" is Latin for "teeth." "Augeo" is Latin for "to enlarge."
Diffindo (spell Harry used to cause Cedric's bag to split apart) - In Latin, "Diffundo" means "scatter."
Ennervate (used to revive someone who has been stunned) - When spelled with two n's, as ennervate, it actually means to add nerve (daring or strength). With one n, as enervate, it means to weaken. The spell in the books is "ennervate," so it makes perfect sense.
Expelliarmus (disarming spell) - Latin combination "to expel" and "arma" (weapon).
Impedimenta (used to slow down attackers) - "Impedio" is Latin for "to hinder."
Imperius Curse - "Impero" is Latin for "to control" and "imperium" is Latin for "absolute control."
Lumos (used to make a wand emit light) - "Lumen" is Latin for "light" and "luminous" means "emitting light" in English.
Morsmordre (makes the Dark Mark appear in the sky) - Combination between "mors" (Latin for "death") and "modere" (Latin for "to bite"). Death bite? No -- Death Eater.
Nox (cancels out Lumos) - "Nox" is Latin for "darkness."
Orchideous (makes flowers spring out of the tip of a wand) - Orchid is a type of flower.
Patronus (dementor defense spell) - "Patronus" is Latin for "protector."
Quietus (cancels out "Sonorus") - Obviously "quiet" means to be silent, and "quietus" in Latin means to be quiet.
Reducio (shrinking charm) - "Reduce" means "to make smaller" in English.
Riddikulus (spell used to transform a scary Boggart into a humorous shape) - "Ridiculous" means "absurd."
Sonorous (spell used to magnify a voice like a loudspeaker) - "Sonorus" is Latin for "loud."
Stupefy (spell used to make someone unconscious) - "Stupefy" in English means "dull the senses of; daze."
Veritaserum (truth potion) - "Veritas" is Latin for "truth" and a serum is a potion.