Star formation feedback, often shortened to feedback, is mechanisms whereby star formation slows itself down, and is used more specifically to refer to electromagnetic radiation (EMR) and stellar wind from stars, which can heat surrounding molecular clouds, leading to the cloud's expansion whereas contraction within such clouds are the mechanism of star formation. Among young stars are the hot, short-lived stars likely to produce this effect. The net result is a long-term limit on the star formation rate (SFR): if it rises high, it will subsequently fall and take some time to resume.
The term feedback is also used for galaxies and galaxy nuclei (galaxy feedback, AGN feedback, quasar feedback) and supernovae (SN feedback), also meaning EMR emissions and/or outflows, which can similarly affect the interstellar medium and slow star formation. For AGNs, the feedback includes jets. Stellar feedback refers to that directly introduced by stars, stellar wind, radiation pressure, and photoheating, and possibly SN feedback, the latter of which is also associated with young, hot stars.
Such feedback (outflows) from many stars can lower the SF rate of the whole galaxy for a time, after which gravity might draw back the expelled gas and the SFR again rises. When the outflows are from stars themselves, this can form a cycle.
Emission lines (e.g., emission line galaxies) can be a sign of feedback. With sufficient heating, e.g., from supersonic jets, excitation and ionization can result, with the subsequent relaxation leading to emission lines. This can be useful for determining the star formation of some very distant galaxies.