Zone of control, abbreviated to ZOC or ZoC, is a concept common to Civilization games, restricting military unit movement close to units controlled by another power. It usually works like this: if a unit is next to a rival unit, it is not allowed to move directly to (but can direct an attack towards) any other tile that is also next to a rival unit, unless one of your units is on it.
In Civilization V, a unit can move directly from one enemy-adjacent tile to another enemy-adjacent tile, but it will lose all movement points, with some exceptions. This applies even if one of that civilization's units is on the tile being moved to. An exception to this is when a melee unit is attacking an enemy in that tile and has the "Can Move After Attacking" ability. If such a unit attacks and kills the enemy unit, it will lose only the normal amount of movement points, ignoring ZoC. An exception to this exception is if the unit being attacked uses a "Withdraw Before Melee" ability, and there is no civilian unit in the attacked tile, which causes the attack to instead be treated as a movement into the tile.
Movement into or out of one of your cities is the standard exception to the rule. Other exceptions depend on the game. In some games, the presence of one of your non-military units will allow the move, but in C-evo it will not. In some games, you can enter an unoccupied rival city, ignoring ZoC, but in C-evo you cannot.