Definition from Wiktionary, the free dictionary
for this particular purpose
special — see special
impromptu — see impromptu
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- On the spur of the moment.
- For a particular purpose.
on the spur of the moment
for a particular purpose
ad hoc (not comparable)
- (rare) ad hoc (for a particular purpose)
Used only in uninflected form to modify a noun, as in ad hoc -komitea ("ad hoc committee").
Usage notes 
The Finnish term mainly used instead of "ad hoc committee" is työryhmä
- to this end, for this, to this point, to this.