Described at
What is a Semibreve? - Answered - Twinkl Teaching Wiki
What is a Semibreve? How is it used differently to other musical note values?
twinkl.co.uk →Link to a page describing this subject · 17,754 chars · not written by Vinony
~2 min read
Encyclopedic overview
Figure 1. A whole note and a whole rest. Comparison of duple note values: = 2×, etc. v t e
A whole note (American) or semibreve (British) in musical notation is a single note equivalent to or lasting as long as two half notes or four quarter notes.
Excerpted from Wikipedia’s “whole note” article, available under the CC BY-SA 4.0 licence.