Webster's English Dictionary

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tenebrae

Ten.e.brae \'ten-*-.bra-, -.bri-, -.bre-\ n [ML, fr. L, darkness] pl but 
   sing or pl in constr  : the office of matins and lauds for the last three 
   days of Holy Week comm emorating the sufferings and death of Christ with a 
   progressive extinguishing of candles