double whole note

US /ˌdʌb.əl ˈhoʊl ˌnoʊt/
UK /ˌdʌb.əl ˈhoʊl ˌnoʊt/
"double whole note" picture
1.

a musical note having the longest time value in common use, equal to two whole notes or eight quarter notes

:
The composer used a double whole note to indicate a sustained sound.
A double whole note is rarely seen in modern music notation.