double whole note
US /ˌdʌb.əl ˈhoʊl ˌnoʊt/
UK /ˌdʌb.əl ˈhoʊl ˌnoʊt/

1.
a musical note having the longest time value in common use, equal to two whole notes or eight quarter notes
:
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The composer used a double whole note to indicate a sustained sound.
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A double whole note is rarely seen in modern music notation.