Perfidy is the act — or the quality — of betraying someone who trusted you. It is the noun that goes with 'perfidious'. You can talk about perfidy in general ('an act of perfidy', 'her perfidy') and it stays uncountable most of the time, though writers do sometimes say 'the perfidies of politicians' when they mean several separate betrayals. Like the adjective, it is formal and literary, and it carries judgement: nobody says 'perfidy' about their own behaviour. It belongs to courtrooms, history writing, sermons, and the kind of newspaper column that is angry on purpose. There is also a narrow, precise use in the laws of war: perfidy means pretending to be protected — waving a white flag, wearing a Red Cross armband, dressing as a civilian — and then attacking. That use is a war crime with a definition, not just a strong word, and it shows the core meaning very cleanly: you use someone's trust as a weapon. In everyday Taiwan-classroom English you would say 'betrayal' instead, which is the ordinary word and is used far more often.

背信、背叛（行為本身）；書面語，日常說 betrayal 較自然