The phrase commensal mode has grown common for a mode in the operation of a telescope such that two different observing goals are achieved simultaneously. This particular phrase has become common with a recent interest in adapting and using radio telescopes to simultaneously carry out a program to survey the sky for static information (e.g., intensity mapping) and for transients that happen to be in the same location. For sensitivity, long observations are needed for static maps of the sky, but for transients, short observations need to be recorded. Since radio telescopes produce an enormous stream of data, more than can be saved in its entirety, the commensal mode strategy means streaming the data into two different mechanisms to process it for recording: one that integrates long observations, and another that triggers on interesting short-term transients (e.g., fast radio bursts, or pulsar pulses, or search for extraterrestrial intelligence monitoring) and records them. Entire radio telescopes (and arrays) with narrow beams necessary to identify sources, sitting waiting for transients is overly expensive. The term piggyback is sometimes used as an alternate term to explain the meaning of commensal mode.
The notion can be used outside radio astronomy and in principle survey work, some kind of individual observation, and transient surveying can make use the same data stream. CCD's and similar data collectors potentially can offer data streams that could be broken down to different time intervals, recording the data in two ways for two goals. There is motivation for any major research telescope, perhaps the highest motivation being for space observatory's with limited lifespans. Slew surveys, surveys collecting data while the telescope is slewed could be classified as a kind of commensal mode.