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In electrical engineering, the term bias has the following meanings:

1. A systematic deviation of a value from a reference value.
2. The amount by which the average of a set of values departs from a reference value.
3. Electrical, mechanical, magnetic, or other force (field) applied to a device to establish a reference level to operate the device.
4. In telegraph signaling systems, the development of a positive or negative DC voltage at a point on a line that should remain at a specified reference level, such as zero.

Note: A bias may be applied or produced by (i) the electrical characteristics of the line, (ii) the terminal equipment, and (iii) the signaling scheme.

(Source: from Federal Standard 1037C and from MIL-STD-188)

Most often, bias simply refers to a fixed DC voltage applied to the same point in a circuit as an AC signal, frequently to select the desired operating response of a semiconductor or other electronic component (forward or reverse bias). For example, a bias voltage is applied to a transistor in an electronic amplifier to allow the transistor to operate in a particular region of its transconductance curve; a (much higher) bias voltage is also often applied to the grid electrodes in a vacuum tube for precisely the same reason.

A hot bias can lower the tube life span, but a "cool" biased can induce crossover distortion.

Bias is also the term used for a high-frequency signal added to the audio signal recorded on magnetic tape. See tape bias.

Bias is used in direct broadcast satellites such as DirecTV and Dish Network, the IRD box actually powers the feedhorn or LNB receiver mounted on the dish arm. This bias is changed from a lower voltage to a higher voltage to select the polarization of the LNB, so that it receives signals that are polarized either clockwise or counterclockwise, thereby allowing it to receive twice as many channels.

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