Here are a few possibilities:
1. Planned: When "ad hoc" means "impromptu" or "unplanned", the opposite is "planned". This refers to something that is organized and thought out beforehand.
2. Systematic: When "ad hoc" means "unstructured" or "unsystematic", the opposite is "systematic". This refers to a method or approach that follows a set of established rules or procedures.
3. Formal: When "ad hoc" means "informal", the opposite is "formal". This refers to something that adheres to established protocols and conventions.
4. Permanent: When "ad hoc" means "temporary", the opposite is "permanent". This refers to something that is intended to last indefinitely.
5. General: When "ad hoc" means "specific", the opposite is "general". This refers to something that applies broadly or universally.
It's important to consider the specific context of the word "ad hoc" to determine the most appropriate opposite.