What does the term "bias" mean?
Bias is another word for the operating point -- a dc voltage or current about which the instantaneous value might vary.

For example, you can say you applied a "6 V peak-peak AC signal biased at +1 V". In this case the range of the signal would be from -2 to +4 V. You can see the relationship with the everyday meaning of bias, "a tendency or inclination" (dictionary.com) in this case with the meaning that although the voltage varies, it tends to be near the operating point.

As the other answers point out, the term is often used with relation to diodes and other nonlinear components.