Overpass is a bomb defusal map that was introduced in the Winter Offensive update along with Cobblestone. It was CS:GO's first completely new defuse map that was designed specifically for use in competitive play. 
Counter-Terrorists: Defend a stalled military shipment on the canal overpass. Terrorists: Attack the shipment head-on or attempt to take down the overpass itself.
- http://blog.counter-strike.net/index.php/2014/04/9284/ Overpass, Evolved