What is Superimposing?

su·per·im·pose/ˌso͞opər-imˈpōz/

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To "superimpose" two things is to add them together. The Superposition Theorem in circuit analysis is a good example of this, where you analyze a multi-source circuit once for each power source, then superimpose all the voltages and currents obtained in those analyses to determine what the sources will do when they're all active at the same time.

A practical example of superposition used in instrumentation is HART circuits, where the DC 4-20 mA analog signal is superimposed with the AC HART FSK "tone" waveforms along the same pair of wires. Using filter circuits we can separate the DC signal from the AC (HART) signal, but they exist together on the same cable as a superposition (a combination of AC plus a DC offset).