Sight is a property of units, structures, and certain summoner spells, items and abilities in the game that, perhaps unsurprisingly, represents a team possessing vision of a target area. Inversely, the area of the map in which a team does not have sight over is known as the Fog of War, a common hallmark of the RTS genre which is represented visually as a dark shroud over the terrain.
Possessing sight is necessary to observe most information about any non-allied units in the game. Such information can include, among many other things, the unit's current location, direction, health, mana, possessed buffs and purchased items, making the acquisition and denial of sight one of the most crucially important skills to learn in high levels of play.
All units in the game will grant vision up to a specific radius. Items and abilities do not always grant sight and those that do often have differing applications of the mechanic to one another.
There exist two obstacles towards sight - impassable terrain (which is always opaque to vision granted by units, structures and most items and abilities) and Brush (which is opaque towards vision when viewed from the outside inwards and not the reverse). Such barriers will also block vision of any areas that are directly behind them from the sight-granting unit. Units that can directly observe an area around these obstacles are commonly described as having line of sight towards that area.
Sight can be removed in three ways - the withdrawal of any sight-granting units, abilities or items from the vicinity, the expiration of a sight-granting debuff or spell, or directly with the Nearsight debuff (which currently only exists with few heroes).
True sight is a unique form of sight linked to the detection of stealthed units. A unit that is stealthed is not affected by standard sight and is only visible with a source of true sight nearby. All sources of true sight currently in the game also provide standard sight - otherwise, true sight follows the same rules as standard sight.
Brush is a common terrain feature found within the game that provides any unit that enters it or the visual effect of AoE skill a form of one-way stealth.
Upon entering any brush, the unit is immediately rendered invisible to enemies outside of that particular patch of brush, a state that is represented when the unit's model takes on a partially transparent quality. The unit will remain invisible as such until it has either left the brush, is revealed by a sight-granting ability, item or an enemy unit, or engages in most kinds of attacks.
Brush does not provide true stealth, as the unit does not require any form of true sight in order to be visible.
All forms of basic attacks and the majority of non-skillshot offensive abilities will reveal the caster and a small area around them for a short period of time, if they are standing in a brush. It is therefore generally impossible to remain permanently stealthed while attacking.
The visual effect of AoE skill cast by having its center located within the brush is also rendered invisible to enemies outside of that particular patch of brush similar to any unit. However, unlike the unit, the enemies outside the brush affected by the AoE skill inside it can't see the visual effect unless the brush that the visual effect takes place is revealed.