Difference between "ad hoc" and "impromptu"

Solution 1:

ad hoc: Created or done for a particular purpose as necessary.
impromptu: Done without being planned or rehearsed

Strictly speaking, Kris' comment is correct. The two expressions have different meanings and are not interchangeable. If every speaker of English would just acknowledge this and use the expressions in their correct sense, the world would be a better place and this question would be off-topic.

However... as with many expression, especially those of foreign origin, the world is not such a beautiful place.

Many people use the two expression interchangeably, and if you are told a meeting room can be used for ad hoc meetings, the speaker usually means any impromptu meeting.

Well, in fact, a lot of impromptu meetings are probably ad hoc:

Guys, the servers crashed again over the weekend, let's get together with the admins and the hardware boys to get to the core of this problem!

If 15 minutes later, there are ten people in a meeting room discussing this problem, they are having both an ad hoc and an impromptu meeting!

On the other hand, you could plan an ad hoc meeting months in advance:

Let's get the CFO's of the twenty biggest companies together next to discuss the impact of new taxation proposals.

That meeting would be ad-hoc, but not impromptu.

Since there is a lot of overlap, it does happen that people get sloppy in the use of the expressions. Of course, I advocate being careful and using the correct expression!

So if you have meeting rooms available that do not need to be booked beforehand, do not reserve them for ad hoc meetings, but for impromptu meetings.

(Personally, I have seen ad hoc used more often in this context!)

Solution 2:

In your example, either is correct, but they do not necessarily mean the same thing. An impromptu meeting is, by definition, not scheduled in advance: the meeting is called, and then it happens immediately or almost immediately. An ad hoc meeting may be impromptu, but not necessarily; what's important is that it's not part of a regularly scheduled series of meetings, but rather, was specifically called. (For example, a certain side project may not have time scheduled for it every week, but whenever enough discussion topics have built up that a meeting would be worthwhile, an ad hoc meeting can be scheduled a few hours or days in advance.)

Both terms convey a certain level of informality; and as a result, both terms must be interpreted relative to the rest of the company's culture, and to the usual scheduling of meetings. For example, if the culture has a very strict bureaucracy, with everyone's schedule generally being booked full of regular scheduled meetings, then even a meeting proposed a day in advance might be described as "impromptu", because by company standards, that's essentially unscheduled. Conversely, at a small company with a very informal culture, where very few meetings are scheduled at fixed regular intervals regardless of circumstances, it's unlikely for any meeting to be described even as "ad hoc", let alone as "impromptu".