1. The superposition of two or more waveforms. Interference is constructive where the waveforms are in-phase (so peaks add to peaks, etc.), destructive where 180° out-of-phase (so peaks tend to cancel troughs). 2. The presence of signals from another source (e.g., atmospheric static) that tend to obscure a desired signal. 3. Presence of one substance interfering with assay for another substance.