From the PHB, page 48, under the “Rage” section:
In battle, you fight with primal ferocity. On your turn, you can enter a rage as a bonus action.
Does this imply that a Barbarian can only enter a rage whilst in combat? And if not, does there need to be an impetus for the rage to occur, or can it be on a whim? For instance, if my Barbarian decides she wants to rage to impress the lord of a manor with her enhanced strength, is that an acceptable condition to start raging?