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  拉丁語片語Deus ex machina(英譯:God from the machine)翻譯自希臘語(ap mkhans thes),意思是機關跑出的神,中文一般翻譯為「舞臺機關送神」、「機械降神」、「機器神」、「解圍之神」等。  在古希臘戲劇,當劇情陷入膠著,困境難以解決時,突然出現擁有強大力量的神將難題解決,令故事得以收拾、有個好結局。扮演神的演員會利用起重機從舞臺上方降下,或是起升機從舞臺地板的活門抬上。這種表演手法是人為的,製造出意料之外的劇情大逆轉。

  這種手法通常被評論家認為是不高明的說書技巧,因為它破壞了故事的內在邏輯,縱使有時候會為了這個理由而故意採用。繼亞里士多德之後,文藝復興時期評論家將其視為一種迂拙的情節計策,雖然其仍然受用於文藝復興時期的劇作家;莎士比亞將此計策用在《》《仲夏夜之夢》《皆大歡喜》《泰爾親王佩力克爾斯》和《冬天的故事》。19世紀末,尼采批評歐里庇得斯透過這種計策將悲劇因素製造成樂觀的型別,並促使他高度懷疑戲劇《blissful delight in life》是「希臘快樂」(Greek cheerfulness)。  尼采認為機械降神是蘇格拉底文化的症狀,重視文化知識過於酒神節的音樂並最終導致死亡的悲劇:  But the new non-Dionysiac spirit is most clearly apparent in the "endings" of the new dramas. At the end of the old tragedies there was a sense of metaphysical conciliation without which it is impossible to imagine our taking delight in tragedy; perhaps the conciliatory tones from another world echo most purely in "Oedipus at Colonus". Now, once tragedy had lost the genius of music, tragedy in the strictest sense was dead: for where was that metaphysical consolation now to be found? Hence an earthly resolution for tragic dissonance was sought; the hero, having been adequately tormented by fate, won his well-earned reward in a stately marriage and tokens of divine honour. The hero had become a gladiator, granted freedom once he had been satisfactorily flayed and scarred. Metaphysical consolation had been ousted by the "deus ex machina".  尼采認為,機械降神製造出虛假的慰藉感覺,這是不應該尋求的現象,這種情節計策的詆譭普遍出現在評論意見中。一些20世紀的修正主義者的評論,建議deus ex machina不能在簡化的用語中被檢視,並主張此種計策是凡人「刺探」與神的關係。拉什瑞姆(Rush Rehm)特別引用希臘悲劇的例子,其中deus ex machina提供複雜化角色的生活和態度,當面臨神的同時為觀眾帶來戲劇的尾聲。
天外救星是意料外的、突然的、牽強的解圍角色、手段或事件,在虛構作品內,突然引入來為緊張情節或場面解圍。近似詞有天降神兵、有如神助等。