What is not explicitly stated so far in the answers is that while a power level should be stated with a reference unit - dBm or dBw or similar, a difference in power levels must not have a unit "I tuned the coil and brought the output up 4 dB"
This is because when you subtract logarithms, what you are actually doing is dividing (the exponential values), and in division the units cancel leaving just a ratio.
So while seeing dB by itself may indicate someone is being sloppy, in well-written engineering documents you will often see a mix of referenced units such as dBm to state absolute powers, and unreferenced ratios in dB to state relative differences.