When referring to sound in the physical world, "decibel" mean dB SPL (sound pressure level). Which is defined as the ratio to the smallest perceivable sound pressure level. Unfortunately that is still a bit underspecified, it may be measured with a frequency weighting like A weighting. And then there is the integration time or other temporal aggregation, but that is separate from decibel/log.

When in analog audio, it usually means dbV, relative to a reference voltage.

And in digital audio, usually dBFS - relative to the maximum amplitude that can be represented.