Impediment Jinx
Incantation: Impedimenta
Pronunciation: im-ped-i-men-tə
Description: This powerful spell 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: Used in Goblet of Fire when Harry is practising for the third task. Also used by Madam Hooch to briefly 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.

Imperius Curse
Incantation: Imperio
Pronunciation: im-peer-ee-oh
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 Crouches, 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: Seen repeatedly throughout the books since it was introduced in GoF.

Imperturbable Charm
Incantation: Unknown
Description: Makes objects such as doors impenetrable (by everything, including sounds and objects).
Seen: The spell is used by Mrs Weasley in Order of the Phoenix on the door of the room in which an Order meeting was being held, to prevent her sons, Fred and George, from eavesdropping using their extendable ears.

Impervius Charm
Incantation: Impervius
Pronunciation: im-pur-vee-əs
Description: This spell makes something repel substances and outside forces, including water.
Seen: 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.

Incarcerous
Incantation: Incarcerous
Pronunciation: ing-kar-sər-əs
Description: Ties someone or something up with ropes which shoot out of the wand.
Seen: Used by Lupin to tie up Snape in the Shrieking Shack in PoA and in GoF when Pettigrew ties Harry to Tom Riddle's grave. Also used by Harry on the Inferi in Voldemort's Horcrux chamber, in HBP, and later again when Harry tries to bound Snape after the death of Dumbledore.

Incendio
Incantation: incendio
Pronunciation: in-sen-dee-oh
Description: Produces fire. Flames burst out flying from the wand.

Intruder Charm
Incantation: Unknown
Description: Detects intruders and sounds an alarm.
Seen: Slughorn had it on a temporary Muggle owned house he was living in, allowing him to detect Dumbledore and Harry as they approached in HBP.

Invisibility Charm
Incantation: Unknown
Description: Renders the object or person invisible.