What is DC signal?

(This answer assumes that this is a straightforward question about the electrical term "DC". If it is about something else, feel free to explain in the discussion area and I will try to answer.) "DC" refers to Direct Current. This is as opposed to "AC", which stands for Alternating Current. Direct Current involves a flow of current (electron flow) in one direction only. The delivery of power to an electronic circuit is by far the most common application of DC current in your house, for example. (There are some high power DC power delivery systems used in industry as well as other non-domestic applications such as welding.) Alternating Current involves a flow of current that reverses direction periodically, that is, at some "frequency". Historically, the vast majority of "signalling" was done with AC. This corresponds to what we commonly call "analog" today, including audio, radio, and telephone communications. With the advent of the digital age, digital signal transmission (in the case of signaling over most wires and circuit traces) could be considered to be switched DC, since the current direction does not reverse, i.e., during the signalling of a digital "one", current flows in one direction, while during the signalling of a digital zero no current flows (so the direction does not alternate, it only stops and starts). (By no means all digital signalling is DC however. All wireless transmission could be considered to be AC, and signalling in systems such as USB and FireWire is AC.)