Modifier

Modifiers are words attached to incantations to alter the effects of a spell, for example, to increase its strength.

Evanesco
The Vanishing Spell is performed by attaching the modifier "evanesco" to the name of the object you wish to vanish. It is unknown if this spell would work if no object was verbally specified.

Extremos
A modifier only ever used in the spell Magicus Extremos, which bolsters the caster's general magical power by pooling the power of themselves and (at least) two other people altogether, forming a magical partnership. It is unknown what the Extremos suffix would do if ever used in conjuction with other spells or if Magicus was used alone

Horribilis
Horribilis has only ever been used in conjunction with the Shield Charm, which allowed it to protect a large area almost completely from the darkest magic, though it is unknown what would happen (if anything) if used with other spells.

Maxima
the Maxima modifier greatly increases the strength of a spell, possibly to its highest intensity.

Uses

 * Bombarda Maxima
 * Lumos Maxima
 * Protego Maxima

Numerical
The numerical modifiers are Duo and Tria. When each is attached to a spell's inantation they increase its strength and can potentially change the spell's appearance, Tria more so than Duo.

Uses

 * Flipendo Duo and Tria
 * Fumos Duo
 * Glacius Duo and Tria
 * Incendio Duo and Tria
 * Lumos Duo
 * Petrificus Totalus Duo and tria
 * Protego Duo
 * Verdimillious Duo and Verdimillious Tria

Solem
This modifier was only ever used with Lumos, which converted its harmless, all-round glow into a harsh ray of light, though it is unknown what would happen (if anything) if used in conjunction with other spells.

Total
The total modifier has two forms (Totalus and Totalum), both coming from the Latin totus meaning "as a whole". It appears to spread the original spell's area of effect.

Uses

 * Petrificus Totalus
 * Protego Totalum

Behind the scenes

 * Although modifiers have appeared many times in the games and films of the franchise, they have never appeared in the books, with the exception of Horribilis.
 * Modifiers appear to be able to be used in conjunction with other modifiers to get the combined effect of both, as the Full Body-Bind Curse (which contains the total modifier) can be used with the numerical modifiers.