A grimoire is a book describing magical beliefs and practices, written between the late-medieval period and the 18th century. Such books contain astrological correspondences, lists of angels and demons, directions on casting charms and spells, on mixing medicines, summoning unearthly entities, and making talismans. "Magical" books in almost any context, especially books of magical spells, are also called grimoires.
The word grimoire is from the Old French gramaire, and is from the same root as the words grammar and glamour. This is partly because, in the mid-late Middle Ages, Latin "grammars" (books on Latin syntax and diction) were foundational to school and university education, as controlled by the Church - while to the illiterate majority, non-ecclesiastical books were suspect as magic. But "grammar" also denoted, to literate and illiterate alike, a book of basic instruction. A grammar is a description of a set of symbols and how to combine them to create well-formed sentences. A Grimoire is, appropriately enough, a description of a set of magickal symbols and how to combine them properly.