A heat shield was a surface or energy barrier designed to prevent damage to spacecraft structures when that spacecraft approached a star or entered the atmosphere of a planet.
When the HMS Bounty attempted the slingshot effect around Sol, Pavel Chekov noted that the heat shields were at maximum moments after the ship hit warp 8. (Star Trek IV: The Voyage Home)
The USS Enterprise-D used its deflector shields when it entered the atmosphere of Minos during a conflict with the Echo Papa 607 weapons system. (TNG: "The Arsenal of Freedom")
The escape pods of Template:ShipClass starships had heat shields that lay flush with the surface of the hull when in place. (Star Trek: First Contact)