decibel

Decibel is built from deci-, meaning one tenth, plus bel, a unit of measurement. Put together, the word literally names a tenth of a bel.

The facts here don't point to a shift in meaning — decibel has simply denoted this tenth-of-a-bel measurement since it was formed.

It shares its second half with the unit bel itself, making bel and decibel direct relatives, one simply a fraction of the other.