Adrenaline Bar
From WikiRaider
The Adrenaline Bar is one of the Status Bars. So far it has only appeared in Tomb Raider Underworld. The bar is located in the upper left corner of the screen, next to the Health Bar. When fighting against enemies, the amount of adrenaline increases and eventually the bar turns completely yellow. Then the player is able to perform a Headshot[1].
It's also possible to perform Concentrated Fire[2] on enemies with the help of adrenaline. This doesn't require that the bar is full, any amount of adrenaline is enough.