Historically the term “spell” is derived from the old books kept by wizards and witches to record the proper grammar and descriptions of various magical terms, names, and procedures. These books were often labelled “Spellings and Grammars”, or simply “Grammars” that were filled with various spellings and similar notes. Over time, evolving as language does, the books themselves came to be known as “Grimoires” and the operations within were eventually and generically called “spells”.
There is also the old German word spel which means story, tale, discourse, command, , narrative, or saying. Quite a word was it not? The modern spell, in magical context, can be any or all of these things: a petition to or communication with causal forces, a story form that lays out exactly what you want to achieve with the magic you are working, a command to subtle energies to do this or that, or an affirmation (on steroids) to create change in yourself or the world around you.
Incantation, a synonym for spell in the current era derives from older words that mean spirited singing, rhythmic chanting, and similar. Indeed, rhyming verses, mantra-like chanting, and various sorts of musical delivery are common methods for delivering the verbal component of magical operations or workings.
When a spell is cast, the magic of the caster is sent with great force, intent, emotion, and willpower into the causal realm, or the dimensions where everything in the manifest world has its true origin. Beyond (in a sense) the three dimensions we know as the material world, the causal plane (some say causal spheres, astral plane, or formative world) is the place ideas pass through as they flow “down” from the mental or spirit plane, in order to begin the process of emerging upon or returning to the material world in the form of a desire or intention fulfilled. If that sounds too “far out” don’t even sweat it I only added the previous blurb for a (semi) thorough explanation.
Over the past century, roughly, the term spell has become entangled and synonymous with any working of magic, such that “magic spell”, “casting a spell” et al are generic terms applied to just about any kind of operation the magician might undertake. Most of the time on this blog when I list a spell it will be, by traditional definition, an incantation, and if otherwise this will be clarified on the accompanying spell key.