Someone; somebody — an indefinite person (used in expressions like 'someone came' or 'is someone there?').
誰か助けてください！
Someone, please help!
昨夜、誰かが電話をかけてきた。
Someone called last night.
ここに誰か忘れ物をしていませんか？
Has someone left something here?
誰かにこの箱を運んでもらえますか？
Could someone carry this box for me?