Battle fever is a euphoric feeling of fearlessness and invincibility, almost akin to feeling drunk, which comes over a warrior fighting in battle. Tyrion Lannister experiences the battle fever for the first time as he leads his troop into the fray of the Battle of the Blackwater.
Tyrion had never thought to experience the battle fever himself, though his brother Jaime had told him of it often enough. As he wields his axe he feels drunk and recalls what Jaime had told him,
How time seemed to blur and slow and even stop, how the past and the future vanished until there was noting but the instant, how fear fled, and thought fled, and even your body. “You don’t feel your wounds then, or the ache in your back from the weight of the armour, or the sweat running down into your eyes. You stop feeling, you stop thinking, you stop being you, there is only the fight, the foe, this man and then the next and the next and the next, and you know they are afraid and tired but you’re not, you’re alive, and death is all around you but their swords move so slowly, and you can dance through them laughing." 
- Archmaester Gyldayn writing of the First Battle of Tumbleton
- Tyrion Lannister, overcome with battle fever as he enters the fray
Battle fever. I am half a man and drunk with slaughter, let them kill me if they can!