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Is there any difference between "ad hoc" and "impromptu"?

Can you find sentences where only one of the words is acceptable and the other is not? And where they are interchangeable?

OED says:

ad hoc adverb & adjective
Created or done for a particular purpose as necessary.

[as adjective] ‘the discussions were on an ad hoc basis’
[as adverb] ‘the group was constituted ad hoc’

impromptu adverb & adjective
Done without being planned or rehearsed.

[as adjective] ‘an impromptu press conference’
[as adverb] ‘he spoke impromptu’

This is still confusing for me, as I often hear to use them interchangeably, like in the following situation:

You have to book meeting rooms in advance, but there are also some meeting rooms available for ad hoc meetings, which you don't have to book.

Is the common use of ad hoc simply incorrect, or has the meaning been shifting recently?

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    Please check the dictionary, and usage examples. The expressions are not interchangeable. – Kris Jun 18 '14 at 6:27
  • @Kris, please read the accepted answer - I simply do not agree with you that the question is off-topic. "Check the dictionary" sounds like an academic alibism to me for words where many people make mistakes. "Since there is a lot of overlap, it does happen that people get sloppy in the use of the expressions." – Honza Zidek Nov 16 '17 at 8:57
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    What matters on this site is that all question askers show their research. Please edit this to explain in detail which dictionaries (or other resources) you have consulted, what they said, and why you are still unclear as to how the words are different. – curiousdannii Nov 16 '17 at 12:44
  • @curiousdannii I have reworded the question, which I should have done before. – Honza Zidek Nov 17 '17 at 15:31
  • @Kris: I am very happy that ELU has questions like english.stackexchange.com/q/23918/73094. One of the good side effects of ELU is that it may serve as a (compared to the rest of internet) relatively authoritative source of references. This question has 26 upvotes, 0 downvotes, many good answers - and could also have been brushed-off by referring the OP to dictionaries. But it was not, and it was a good decision. The only difference is that that question comes from the good old times of ELU six years ago when the whole Stackexchange used to be a friendly place. – Honza Zidek Nov 17 '17 at 22:16
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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!)

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    I'd suggest that dictionaries lag the spoken word which is constantly changing and subjectively different for each speaker; language evolves through interpretation. Indeed, I assert that this fluidity has an intrinsic beauty, without it, how would we describe new ideas. A fixed and unchanging language would be dreary at best. – Jodrell Jun 18 '14 at 13:07
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    @Jodrell: I full-heartedly agree with you, but there is a difference between shifting semantics or application of new meanings (a mouse became an electronic device), and a merger of expressions because people forget their original meaning. Yes, the latter happens, but I don't see how a new idea emerges, it is rather a sign of a language losing something. – oerkelens Jun 18 '14 at 13:10
  • The meaning of mouse has not been conflated, organic mice and timid people still exist. Take, for example, The term unlike which has been understood since at least 1761 (Memoirs of Miss Sidney Bidulph) “I cannot compel it to like, and unlike, and like anew at pleasure.” but now social media has given it new purpose. – Jodrell Jun 18 '14 at 14:30
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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".

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The impromptu chicken coop I built this past weekend, resulted in a lot of ad hoc modifications....

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