hundred battles; many battles
Almost always used in fixed expressions such as 百戦錬磨 (veteran of many battles) or 百戦百勝 (ever-victorious). Rarely used as a standalone noun in modern Japanese.
百戦錬磨 means being tempered through countless battles.
百戦百勝 expresses winning every single battle.
From 百 (hundred) + 戦 (battle). The exact historical derivation is uncertain; the spelling is conventionally associated with the meaning 'many battles'.