I

Impediment Curse ~ To slow down and obstruct attackers; impedimenta is the incantation. [4,31]

Imperio ~ The Imperius curse; gives a wizard total control over another life. An Unforgivable Curse. [4,14]

Imperturbable Charm ~ Renders an object safe from disturbance - or spying. [5,4]

Impervius ~ A charm to make an object repel water. [3,9]

Incarcerous ~ A spell that causes ropes to secure an object - or person. Or creature. [5,33]

Incendio ~ A cahrm to start a fire. [4,4]

Inferius ~ A corpse bewitched to do a Dark Wizard's bidding; inferi were used by Lord Voldemort, who "killed enough people to make an army of them". [6,3]

Inquisitorial Squad ~ A select group of students, supportive of the Ministry for Magic , hand-picked by Professor Umbridge. [5,28]

Invisibility Booster ~ Device to ensure invisibility, of a Ford Anglia, for example. [2,5]

Invisibility Cloak ~ Allows wearer to proceed unseen; except for certain wizarding devices that might reveal the wearer. And cats, possibly. [1,12]