dB is an abbreviation for "decibel". One decibel is one tenth of a Bel, named for Alexander Graham Bell. The measurement quoted in dB describes the ratio (10 log power difference, 20 log voltage difference, etc.) between the quantity of two levels, the level being measured and a reference. To describe an absolute value, the reference point must be known. There are a number of different reference points defined. Here are a few: