Line of sight (sometimes referred to with the acronym, LOS).
Line of sight is an unobstructed line drawn from your character to an enemy. Most abilities or spells and ranged attacks require that you have line of sight to your target to cast them, and will give a message when you don't, canceling the activation.
When casting a channeled spell on an aggroed enemy, when that enemy moves in such a way as to block your LOS at any point in that cast, it will cancel your spell. For example, if you are on the ground, and you begin casting Smite or any such spell on a melee mob that is in an elevated shack, when that mob comes at you, he will cross behind pillars, most likely, and at that point your cast will be canceled. (For those that recognize the region, the islands in the southeast of Durotar have such shacks.) For channeled spells, however, the mob may not be attacked one second or another, or perhaps the effect that the spell gives (such as Mind Flay) may not proc, but the next tick where there is no obstruction during that span, it will continue.
Obstructed LOS is often inconvenient, forcing the player to move to regain LOS before attacking an enemy or healing an ally. One can also intentionally obstruct LOS, e.g. by running around a corner, to force an enemy to move to a more advantageous location.
- Uneven terrain, staircases, walls, and other environmental details can obstruct LOS, causing ranged actions to fail.
- Line of sight appears to be drawn from midsection to midsection, so short characters get the "No Line of Sight" message slightly more often.
- Terrain (that is, dirt and trees) only block Line of Sight indoors. All dirt and most trees do NOT block spells or abilities when fighting outdoors.