A Stranger (未確定存在 (ストレンジャー) Mi-kakutei Sonzai (Sutoranjaa) "Uncertain Existence (Stranger)") is someone who intentionally enters a Phantom Field from from outside it.
Not to be confused with Contaminants, which describes any existence that doesn't belong to the original world the Phantom Field was constructed from (or based off of), and can include existences that were "pulled" into the Phantom Field after its creation, not just those that Dive in from outside.
Kokonoe uses different readings when she talks about Strangers than Raabe does (未確定因子 (ストレンジャー) Mi-kakutei Inshi (Sutoranjaa) "Uncertain Factors" or 異邦人 (ストレンジャー) Ihoujin (Sutoranjaa) "Foreigner (Stranger)").
Because the status of being a Stranger is relative to the Phantom Field, members change depending on the Phantom Field, but consistent Strangers throughout Dark War include the members of Babel, Rei, Ciel Sulfur, and Raabe. Rei and Another Dark Mai are explicitly named Strangers by Kagami Hajou.