The Ice Barrier is a sector of Lyoko. It is sometimes referred to as the Polar Region (because it is covered in ice) or Sector 1 (as referred to in Mister Pück). It also has Virtual Weather, as Jeremie pointed out in The Girl of the Dreams and in a way Odd in Cold War.
The Ice Barrier is made up of two parts. The first part is the surface, composed of glaciers connected by narrow paths and large lakes of icy water. Monsters cannot swim, so the icy waters will destroy them if they fall into it. Most of the Towers are based on the surface. The second part of the sector is below the surface; tunnels connect the two areas. Team Lyoko often has to surf down these tunnels to get to an activated Tower.
The Ice Barrier is home to all kinds of monsters, such as Blocks, Hornets, and Mantas. Due to its difficult range of reachable locations, XANA placed his Boulogne-Billiancourt simulation program there in Ghost Channel.
In season 1 this sector used to have a strange light source that was always lighting and shadowing Team Lyoko's faces. It is most likely that it is Carthage's shining outer shell, but it is unknown why the light was taken away in season 2.