Original: An analog signal is a datum that changes over time -- say , the temperature at a given location ; the depth of a certain point in a pond ; or the amplitude of the voltage at some node in a circuit -- that can be represented as a mathematical function , with time as the free variable ( abscissa ) and the signal itself as the dependent variable ( ordinate ) .
Simplified: An analog signal is a datum that changes over time ( for instance the temperature at a given place ; the depth of a certain point in a pond ; or the amplitude of the voltage at some node in a circuit ) .