A terraforming mission was a mission with the objective of terraforming a planet.
In 2368, USS Enterprise-D's terraforming mission to Doraf I was cancelled due to Ambassador Spock's disappearance and possible defection to the Romulans. (TNG: "Unification I")
In 2369, the USS Gandhi, Thomas Riker's new posting, was heading on a terraforming mission to the Lagana sector. The trip alone would take four months. (TNG: "Second Chances")