What does bias mean in an electrical circuit?

Bias is a permanent voltage, applied to the input of an amplifier device, in order to make it work in the correct area of an amplification curve. Valves (thermionic tubes) and semiconductors (transistors) both use bias in their grid or base connection. A negative swing in the input could put the control input below the working of the device and cause it to switch off. This is undesirable as it will show as distortion. The bias raises the zero point, so that the signal will not turn the device off.