Overwatch (EU2012)

A soldier or alien with a remaining action can enter Overwatch (default hotkey Y). This ends the soldier's turn, but allows them to shoot at the first enemy that enters their overwatch range during the enemy turn, albeit at a small penalty(aim penalty/cannot crit).

Some class abilities alter or improve various aspects and properties of Overwatch. These include Rapid Reaction (Heavy), Squad Sight (Sniper), Opportunist (Sniper), Covering Fire (Support), and Sentinel (Support).

When overwatching be aware of what accuracy percentage you are likely to get. It is better to hunker down and double your defense bonus rather than risking a 20% shot. Especially early on with rookies with low accuracy and carrying weak weapons it isn't worth the risk.