An incantation is both a word that can refer to a spell as well a word referring to the spoken, verbal formula used for performing a spell. One may regard "incantation" as both referring to any spell that requires a verbal formula to work, or to refer to the words for inidvidual spells, regardless of the type of spell (I.E. charm, hex etc.). Most incantations are in Latin though English ones do exist, most of which are in rhymes.
Incantations are very important in casting spells successfully, and messing the words up or interrupting an incantation, or adding your own words to someone else's incantation, while it is in proccess of being cited, may even change the effect of a spell, cause it to not work properly or not work at all, or may even cause more fatal or even lethal consequences depending on what spell is being attempted. For example, while Ana Mahgo cast a spell on Lord Bane's crystal ball to cause anyone who tries to move it or steal it to not be able to do so, Brendan Mahgo interrupted her chant and added his own words, which eventually caused Clara Towers to be teleported away when she tried to take it, but wirh different parts of her body being teleported to separate places, killing her.