Answer: Pause
In musical meter and poetry, a caesura is a complete pause in the composition or line, respectively. In Western sheet music, it appears as two vertical slashes, similar to, but significantly thicker than, the forward slashes found in the beginning of website URLs like //. When used in poetic notation, the slashes are vertical like ||, which looks mighty familiar.
This particular parallel notation found life off the page with the advent of musical control interfaces. Designers, searching for an intuitive way to indicate the music would be paused when the button in question was pushed, labeled it with a slightly modified version of the musical notation for pause itself, the caesura. The use of the slashes in such a fashion appears as far back as the 1960s, but came to prominence in the 1980s through the present as home and portable music and media devices proliferated.