Engorgio
Makes something grow larger. Seen in Harry Potter and the Goblet of Fire when Professor Moody casts it on a spider to enhance a demonstration of the effects of an Unforgivable Curse.
Ennervate
The counterspell to the Stunning Spell. In Harry Potter and the Goblet of Fire, Amos Diggory uses it to wake up Winky.
Expecto Patronum
This spell conjures a Patronus, a silvery phantom shape, usually that of an animal, which is the embodiment of the positive thoughts of the caster. A Patronus will drive away Dementors and Lethifolds. Harry conjures a stag as his Patronus, while Hermione Granger conjures an otter and Cho Chang a swan. In order for the spell to work, the person must concentrate on a happy memory, which is especially difficult in the presence of a Dementor. Literally translated "expecto patronum", which is Latin, means "I expect a guardian"
See also Patronus Charm.
Expelliarmus
This spell is used to disarm another wizard, typically by causing the victim's wand to fly out of reach. As demonstrated in the novel Harry Potter and the Prisoner of Azkaban, simultaneous use of this spell by multiple people on a single wizard can be powerful enough to knock back the wizard himself. First seen in Harry Potter and the Chamber of Secrets, when Snape disarms Lockhart in the Duelling Club.
Evanesco
Makes something vanish. Used in Harry Potter and the Order of the Phoenix when Snape makes Harry's Draught of Peace disappear from his cauldron.
Featherweight Charm
Makes something lightweight. Harry contemplates using this in Harry Potter and the Prisoner of Azkaban to lighten his trunk so he can carry it by broom to Hogwarts. Before he uses it, however, he accidentally summons the Knight Bus.
Ferula
Creates a splint. Used by Remus Lupin in Harry Potter and the Prisoner of Azkaban to bind Ron's leg in the Shrieking Shack.
The Fidelius Charm
This complex charm enables secret information to be hidden within a single living soul of someone known as a Secret-Keeper. The information is then irretrievable until and unless the Secret-Keeper chooses to reveal it; not even those who have the secret revealed to them can reveal it to others. It originates from the Latin word "fidelis", which means "faithful" or "trusting".
So far, there have been only two uses of the Fidelius Charm in the Harry Potter series. In Harry Potter and the Prisoner of Azkaban, it is explained that when Harry was an infant, he and his parents, Lily and James, were hidden from Lord Voldemort by this charm. They initially selected Sirius Black to be their Secret-Keeper, but Black recommended that they choose Peter Pettigrew instead. When they did, Pettigrew betrayed them to Voldemort. In Harry Potter and the Order of the Phoenix, the charm is used to hide the headquarters for the Order of the Phoenix; Albus Dumbledore is the Secret-keeper, and is apparently able to use a letter to reveal the information. This implies that a direct, personal act is not required to bring a new person into the secret.
Finite Incantatum
Stops the effects of spells. Snape uses it in Harry Potter and the Chamber of Secrets to restore order in the Duelling Club.
Flagrate
With this spell, the caster's wand can leave firey marks usually in the shape of an "X". Used by Hermione in Harry Potter and the Order of the Phoenix in the Ministry of Magic.
Furnunculus
Makes boils appear on the victim. Used in Harry Potter and the Goblet of Fire when Harry and Draco Malfoy get in a fight and Harry hits Gregory Goyle accidentally.
Impedimenta
This hex makes something slow down momentarily. Used in Harry Potter and the Goblet of Fire when Harry is practising for the third task.
Impervius
Makes something repel water. Used by Hermione in Harry Potter and the Prisoner of Azkaban on Harry's glasses and also by the Gryffindor Quidditch team in Harry Potter and the Order of the Phoenix.
Incarcerous
Ties someone or something up with ropes. Seen in the Harry Potter Trading Card Game. This incantation is not seen in the books, but a similar spell (with no incantation given) is used to tie up Peter Pettigrew in Harry Potter and the Prisoner of Azkaban.
Incendio
Makes a fire. Used in Harry Potter and the Goblet of Fire by Arthur Weasley to create a fire for Floo powder. In the video game based on the first book, this spell is used to temporarily stun dangerous plants.