Short answers, pulled from the story.
A battle is defined as combat between large components of forces in a military campaign used to achieve military objectives. English historian John Keegan suggested an ideal definition as something happening between two armies leading to the moral and physical disintegration of one or both sides.
The English language borrowed the word battle from Old French around 1297. It traces back through Late Latin to mean exercise of soldiers and gladiators in fighting and fencing.
Naval battles have occurred since before the 5th century BC using fast ships with battering rams to sink opposing fleets. Ancient navies also steered close enough for boarding in hand-to-hand combat where troops stormed enemy ships.
Henry V of England defeated a French army on the 25th of October 1415 during the Battle of Agincourt. He met with the senior French herald who agreed to name the battle after the nearby castle.
Personal effects of battle range from mild psychological issues to permanent and crippling injuries including nightmares, flashbacks, and abnormal reactions to certain sights or sounds. Physical effects can include scars, amputations, lesions, loss of bodily functions, blindness, paralysis and death.