Spell

A spell is a magical action often accompanied by an incantation (usually - but not always - Latin based; for example, Obliviate or Point Me) that makes use of the magical force of the wizarding world in order to accomplish extraordinary feats ordinary people cannot perform, for example levitating objects, conjuring fire, or stunning a person. Spells often have some kind of physical representation in the form of light and as such can usually miss, be dodged, or blocked with an object (including another spell). The action of performing a spell is referred to as casting.

Usage
"Now, don't forget that nice wrist movement we've been practicing! The swish and flick! And saying the magic words properly is very important, too - never forget Wizard Baruffio, who said 's' instead of 'f' and found himself on the floor with a buffalo on his chest."

- Professor Flitwick in Charms Class

Normally, spells require three main factors to be properly accounted for in order to succeed: wand movement, incantation and intent. If one does not move their wand in the correct way, does not speak the incantation properly and/or cannot retain their desired outcome in their imagination during casting then the spell will fail or backfire.

It should be noted that spells can be performed with or without speech and wands, though non-verbal and wandless spells are much more difficult and hence are usually not taught to practitioners until they reach a more advanced level, such as the sixth year of study at Hogwarts. Notable practitioners of non-verbal spells include Albus Dumbledore, Lord Voldemort, and Severus Snape, who have also been known to perform wandless spells.

Most incantations derive from the Latin language (such as Accio) but there are also some English incantations (such as Obliviate or Stupefy).

A simplified analogy for the differentiation between Transfiguration and Charms is the difference between form and function or matter and energy; one is concerned with what the object is (its form - Transfigurations) whilst the other is concerned with what the object is doing (its function - Charms).

Appearance
When performed, spells usually evoke light from the wand they are cast with, such as the tell-tale red of Stupefy or green of Avada Kedavra. However, some spells, such as Accio and Silencio, produce no visible intermediate effect. Some spells also make loud noises when cast, although Harry Potter noted that "bangs and smoke were more often the marks of ineptitude than experience."

Durations
Typically, spells brought about through magic have a certain time limit placed upon them. This means that effects and after-effects magically induced by them are not meant to last, since spells like Expelliarmus only operate brief and swift, while spells like Stupefy and Petrificus Totalus wear off eventually. Counter-curses can also end a designated spell's effect. Likewise, a spell whose effect has yet to diminish would disappear at the moment of the caster's death.

Origin
New spells can be crafted by wizards and witches, though it is implied to be a difficult and dangerous practice, as exemplified by Luna Lovegood's mother, who was killed in a backfired attempt at spellcrafting. Severus Snape is often attributed with the creation of spells such as Levicorpus and Sectumsempra during his time as a student at Hogwarts, though whether he actually crafted them intentionally, discovered them by accident, or learned of them from an outside source is unclear.

Classification
Below is a list of all known categories that spells can be assigned to. It should be noted that the extent to which the following categories are mutually exclusive to each other is not known; i.e. whilst a spell can't be both a charm and a transfiguration it isn't known if a jinx could also be classed as a charm. It also seems likely that certain spell types belong to separate classification schemes to others. For example: Petrification is dark magic of the most advanced kind but it is also Transfiguration, Melofors is both a Jinx and a Conjuration, Fiendfyre could be considered a Charm as well as a curse and many healing spells could also be considered either charms or transfigurations. Hence, it appears that charm vs. transfiguration is one method of classification and counter-spell vs. jinx/hex/curse vs. healing spell a different one.

A simplified analogy for the differentiation between a transfiguration and a charm is the difference between form and function or matter and energy; one is concerned with what the object is (its form - transfigurations) whilst the other is concerned with what the object is doing (its function - charms).

Behind the scenes

 * J. K. Rowling defined a spell as "The generic term for a piece of magic."

Appearances

 * Harry Potter and the Philosopher's Stone
 * Harry Potter and the Philosopher's Stone (film)
 * Harry Potter and the Philosopher's Stone (video game)
 * Harry Potter and the Chamber of Secrets
 * Harry Potter and the Chamber of Secrets (film)
 * Harry Potter and the Chamber of Secrets (video game)
 * Harry Potter and the Prisoner of Azkaban
 * Harry Potter and the Prisoner of Azkaban (film)
 * Harry Potter and the Prisoner of Azkaban (video game)
 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Goblet of Fire (video game)
 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Order of the Phoenix (film)
 * Harry Potter and the Order of the Phoenix (video game)
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Half-Blood Prince (video game)
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 1
 * Harry Potter and the Deathly Hallows: Part 1 (video game)
 * Harry Potter and the Deathly Hallows: Part 2
 * Harry Potter and the Deathly Hallows: Part 2 (video game)
 * The Tales of Beedle the Bard (real)
 * LEGO Harry Potter: Characters of the Magical World
 * LEGO Harry Potter: Building the Magical World
 * The Queen's Handbag
 * LEGO Harry Potter: Years 1-4
 * LEGO Harry Potter: Years 5-7
 * LEGO Creator: Harry Potter
 * Harry Potter and the Forbidden Journey
 * Harry Potter and the Deathly Hallows: Motorbike Escape
 * Harry Potter LEGO Sets
 * Harry Potter for Kinect
 * Wonderbook: Book of Spells

Notes and References
Loitsu Sortilège (définition)