名詞
參與犯罪但願意作供指證同伴嘅人，有時係為咗換取減刑或撤控（量詞：個） suspect turned prosecution witness, sometimes in return for favorable treatment by the prosecution
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污點証人
Definitions (Wiktionary)
noun
accused or convicted criminal who testifies as a witness for the state against his/her associates or accomplices