A decibel is a unit for measuring how loud a sound is. People write it dB. A soft whisper sits low on the scale; a motorbike or a concert sits much higher. The scale is not like a ruler in centimetres. If something is ten decibels louder, the jump is bigger than the number makes it look, because the scale is built in steps of ten. Scientists also use decibels for other kinds of signal, like radio, but in everyday English it is almost always about sound. People stretch it too: “Could you take it down a few decibels?” just means “please make it quieter,” and nobody is holding a meter. One decibel is one tenth of a bel, an older unit you will almost never hear on its own.