Although the decibels unit apparently first arose to describe a power ratio, and is still used that way, it has since also become well established as a separate unit to describe a voltage ratio, independent of the impedances involved. This is discussed in the Wikipedia article:
The IEC permits the use of the decibel with field quantities as well as power and this recommendation is followed by many national standards bodies, such as NIST, which justifies the use of the decibel for voltage ratios.
For the "field quantity" use of decibel, such as for voltage, the "20" form of the equation is used: 20 log (V1/V2).
If two signals are compared, and the impedance seen by them is equal, then the voltage ratio as reported in dB units will be the same as the power ratio in dB. But it is fine to use dB to report a ratio of voltages where the impedance is different and hence the power ratio in dB would not be the same as the voltage ratio in dB. Obviously it's important to be clear in the surrounding description which ratio is being reported.
(The terminology "field quantity" was found misleading, and was revised to "quantity whose square is, in general, proportional to power".)
To be clear, the answer I'm giving here disagrees with Olin's in the sense that his regards the voltage ratio use of dB as a side effect of the power meaning of dB plus the assumption, even if not true, that the two voltage signals are in the presence of equal impedances. In contrast, my answer regards the power and voltage dBs as two separate and equally legitimate units with their own distinct rules for application.