Spells - I

Impedimenta (Impediment Jinx, Impediment Curse)

Pronunciation: im-ped-ih-MEN-tah (IPA: /ɪm.ˌpɛ.dɪ.'mɛn.ta/

Description: This hex is capable of tripping, freezing, binding, knocking back and generally impeding the target's progress towards the caster. The extent to which the spell's specific action can be controlled by the caster is not made clear. If this spell does bind it does eventually wear off as stated in Deathly Hallows.
Seen/Mentioned: Used in Goblet of Fire when Harry is practicing for the third task. Also used by Madam Hooch to shortly stop Harry from fighting with Draco. Also seen toward the end of Order of the Phoenix, when Harry is fighting the Death Eaters. Stronger uses of this spell seem capable of blowing targets away.
Suggested Etymology: Latin impedimentum (plural impedimenta) meaning "a hindrance" or "an impediment".

Imperio (Imperius Curse)

Pronunciation: im-PEER-ee-oh (IPA: [ɪm.'pi.ɹi.ˌəʊ]) (classical Latin: eem-PEHR-ee-oh (IPA: [im.ˈpɛɾ.i.ˌɔ])

Description: Causes the victim of the curse to obey the spoken/unspoken commands of the caster. The experience of being controlled by this curse is described as a complete, wonderful release from any sense of responsibility or worry over one's actions, at the price of one's free will. Resisting the effect of the curse is possible, however, and several individuals have been able to successfully overcome it, including Harry and both of the Crouchs, who learn to resist the curse after being subjected to its effects for an extended period. Harry describes the feeling of being the caster as controlling a marionette through a wand (although Harry's particular experience is suspect due to his lack of commitment to casting Unforgivable Curses). One of the three Unforgivable Curses.
Seen/Mentioned: first mentioned (not by name) in the first book when Ron told Harry that Lucius Malfoy claimed himself being jinxed during the first war, thus Lucius evaded capture. First seen in Goblet of Fire introduced by Barty Crouch Jr (acting as Moody) and used on a spider. Used by Harry in Deathly Hallows on a Gringotts goblin and a Death Eater.
Suggested Etymology: Imperare is Latin for "to order, command", and is the root of several modern English words. Imperium means "command" or "domain", and imperio means (among other things) "with authority". (Compare to impero, "I command", and to crucio above.) Imperius is not, however, a Latin word.

Impervius (Impervius Charm)

Pronunciation: im-PURR-vee-uss (IPA: [ɪm.'pɝ.vi.ˌɛs])

Description: This spell makes something repel (literally, become impervious to) substances and outside forces, including water.
Seen/Mentioned: Used by Hermione in Prisoner of Azkaban on Harry's glasses while in a Quidditch match and also by the Gryffindor Quidditch team in Order of the Phoenix, both times to allow team members to see in a driving rain. Also used in Deathly Hallows, first by Ron to protect objects in Yaxley's office from rain, and then by Hermione in an attempt to protect Harry, Ron and Griphook from the burning treasure in the Lestranges' vault.
Suggested Etymology: Latin impervious meaning "impassable".

Incarcerous

Pronunciation: in-CAR-ser-us (IPA: [ɪn.'kaɹ.sɝ.ˌɪs])

Description: Ties someone or something up with ropes.
Seen/Mentioned: First heard in Order of the Phoenix, when Umbridge gets in a battle with the centaurs. Also used by Harry on the Inferi in Voldemort's Horcrux chamber, in Half-Blood Prince.
Suggested Etymology: English incarcerate meaning "to imprison" or "to confine".

Incendio

Pronunciation: in-SEN-dee-oh (IPA: [ɪn.'sɛn.di.ˌəʊ])

Description: Produces fire.[20]
Seen/Mentioned: It is first seen in Harry Potter and the Philosopher's Stone when Hagrid (non-verbally) produces fire out of his umbrella in the little house the Dursleys took refuge in (from the Hogwarts letters). In Half-Blood Prince, this spell is used several times in battle, for instance when Hagrid's hut is set ablaze.
Suggested Etymology: Latin incendo meaning "to set fire"