以上术语均与“第三人诉讼” （third party proceedings或 third party action)中的“第三人”相关。其中，third party (也称为outside party，third person)为“诉讼第三人”，泛指经被告申请后进入本诉（this action)审判程序但却不属于本诉主要当事人（principal party)的人或实体[1]。其作用主要是分担（contribution) 或补偿（imdemnity)被告在诉讼中的损失。

在诉讼中，被告为保护自己的利益，可用impleader的方式，对原本不是本诉被告的第三人提起诉讼主张（claim)，使之成为第三人之诉(third party proceedings)的被告。这种由被告追加成为诉讼当事人的第三人则被称为impleaded party (被告追加的诉讼第三人）或third party defendant (第三人之诉的被告）[2]。

此外，第三人也可自愿参与诉讼，成为intervener (或intervenor)(“ 参与诉讼者”）或 amicus curiae (或 friend of the court) (“法庭之友”）。“参与诉讼者”与“法庭之友”之区别在于前者完全是因为利益相关而自愿介人诉讼，而后者则可能是自愿或经法院要求而参与诉讼[3]。凡以intervenor身份参与诉讼的形式被称为 intervention 。

[1]“Someone other than the principal parties, also termed outside party. ”Cf. Bryan A. Gamer, Black's Law Dictionary, 7th edition, at p. 1489，West Group (1999).

[2]“In order to provide defending parties an opportunity to more fully protect themselves , most procedural systems provide some mechanism by which the defendant can bring in (implead) a third party defendant (impleaded party). ”Id. at p. 116.

[3]“An intervenor is a person who is not already a party to an ongoing action but who seeks voluntarily to be made a party, typically because she shares some interest in the litigation and is concerned that in her absence that interest will not be adequately protected. ”Id. at p. 118. Amicus curiae is“ a person who is not a party to a lawsuit but who petitions the court or is requested by the court to file a brief in the action because that person has a strong interest in the subject matter.”Cf. Bryan A. Gamer, Black's Law Dictionary, 7th edition, at p. 1489, West Group (1999).

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