1. a device or material that allows some elements of a mixture (e.g., of light, a liquid, or a gas) to pass through but not others. In acoustics, a low-pass filter passes all components below a specified cut-off frequency while attenuating higher frequencies, thus altering the amplitude spectrum of the input; a high-pass filter performs the opposite spectral alteration. A bandpass filter transmits the range of frequencies (passband) lying between specified high-pass and low-pass cut-off frequencies.

2. a hypothetical construct applied to cognitive channels of information that allow only certain aspects of a stimulus to pass into sensory consciousness. Filter metaphors aim to explain the ability to focus selectively on local and precise aspects of the environment (e.g., a conversation in a noisy room).

3. any analytical procedure used in time-series analysis to remove fluctuations from the data and separate out its trend and cyclical components.