A spell can be many things, but this is usually said in reference to a set of words or verse suggestive of witchcraft.
The word spell is inherited from Germanic; however, the definition relating to the occult didn't come to be until Early Modern English around 1579 CE. The oldest definition relates to a subject of discourse, but became obsolete during the Early Modern English era. Spelling variations were spel and spelle. These spellings were common amongst many of the Germanic languages, such as Old Saxon, Middle Dutch, and Gothic. This is a separate entry than spell like how to spell words that came about around 1400 CE.
To cast a spell, one must speak or sing their intentions into the world. It is said that speaking words into existence will assist in that manifestation. This is supported by the idea that words have power. Many spells require additional physical components such as crystals or herbs. This tends to be the majority of what exists within a grimoire.
Sources:
“Spell, N. (1).” Oxford English Dictionary, Oxford UP, September 2023, https://doi.org/10.1093/OED/1166128721.
“Spell, V. (2).” Oxford English Dictionary, Oxford UP, March 2024, https://doi.org/10.1093/OED/2524972481.