A demonstrative prenominal meaning 'that' for something distant from both the speaker and listener or otherwise unfamiliar to both. Used before nouns (e.g., あの人, あの本).
あの山は富士山ですか？
Is that mountain over there Mount Fuji?
あの人は誰ですか？見たことがない顔だ。
Who is that person? I haven't seen that face before.
あの件についてはまだ情報が少ない。
There is still little information about that matter.