(grammar) The practice of specifying what types of complements a word may take when that word acts as the head of a phrase of a certain category; this specification restricts the wordʼs category, making it smaller, hence a “subcategory” of the wordʼs category.
(category theory) A subclass of a category which is itself a category, whose arrows are a restriction of the arrows of the parent category, and whose composition rule is a restriction of the parent category's