A signal whose value can vary continuously over time rather than taking a fixed value. For example, when someone speaks, the sound wave is an analogue signal; it varies smoothly as the person speaks. In contrast, a gear box is a digital device; a car can be in first, second, third or reverse but not in "first-and-a-half". PCs will only work with numbers so cannot directly deal with analogue signals. In order to get around this, you need to fit an analogue-to-digital convertor (A/D convertor). For example, a sound card contains an analogue to digital converter to convert the sound signal from the microphone into numbers representing the volume.
