Ad hoc, a loanword from Latin (where it means, literally, to this), means for this specific purpose. It usually functions as an adjective preceding the noun it modifies—for example:
Every inch of green grass outside the mall had been transformed into ad hoc parking lots. [Dallas Morning News]
An ad hoc committee voted unanimously Wednesday to recommend the city implement cell phone restrictions in school zones. [Amarillo Globe-News]
Bagley is the chief transportation planner for Montgomery County and part of an ad hoc mobility task force that sprung up when the bridge closed in October … [Pottstown Mercury]
Often, verbose phrases involving ad hoc could be shortened—for example:
We have been collaborating on an ad hoc basis with [collaborating ad hoc with] Ignacio for some time. [Fresh Plaza]