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?
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?