Ad hoc meetings are one-off meetings. They are usually about a specific topic or discussion versus a recurring meeting that takes place at regular intervals and has ongoing topics. Ad hoc meetings can be unstructured with no preparation/planning, or they can be structured with an agenda and meeting minutes. It depends on the purpose of the meeting.
Why have an ad hoc meeting?
Most businesses have regular recurring meetings that happen weekly, monthly, or quarterly. You would have an ad hoc meeting when something comes up that can't wait until the next regular meeting. It is usually when something requires decision or action urgently.
Examples of ad hoc meetings
Unstructured ad hoc meeting: You are going on vacation next week so you have an ad hoc meeting with the person who is covering your workload. You want to make sure they know what they are supposed to do. You don’t really need to put together a planned agenda or take meeting minutes, it is a discussion.
Structured ad hoc meeting: You hosted a 10th-anniversary event for your business with clients and colleagues and are doing a post-event review/retrospective to talk about how it went. The event is not happening again but you want to talk with your team about the budget, get feedback on the logistics, and hear about any valuable client interactions that will require immediate follow-up. You would probably put an agenda together and have minutes taken so you can take action on the feedback.