This is called 'Ritardando'
Unfortunately I couldn't find a symbol for you ):
The term ritardando or rallentando are the two most common ways of writing that.
This is the rest symbol in Music
The musical symbol meaning to emphasize a note is called an accent.
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The musical clef symbol is important because it tells musicians which notes correspond to which lines and spaces on the staff, helping them determine the pitch of a musical note.
It's the musical rest symbol.
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The sharp symbol (#)
The simile music symbol in musical notation indicates that the previous section of music should be repeated.
Staccato - .
The purpose of the breath mark symbol in musical notation is to indicate to the musician where to take a breath while playing or singing, helping to maintain proper phrasing and musical expression.