In astronomy, quenching refers to the shutting-down of star formation within a galaxy. A galaxy where star formation has quenched is known as a quenched or quiescent galaxy. Quenching is an important phenomenon in the study of galaxy evolution, as all galaxies can be divided into two fundamental types: actively star-forming or quenched.

Compared to a star-forming counterpart, a quenched galaxy tends to be redder in the visible spectrum and contain older stellar populations, a direct consequence of its star formation being shut off. Most elliptical and lenticular galaxies known to date have these features, which, along with their weak star formation, qualify them as quiescent. Additionally, quenched galaxies also exist in more massive dark matter halos and can be found in denser environments, such as clusters or groups.

Until recently, most quenched galaxies have been found in the local Universe. Since the late 2010s, deep-field surveys in near-infrared bands, including some by the James Webb Space Telescope, have found a number of quenched galaxies in the early Universe. Various mechanisms have been proposed as drivers of quenching, but their relevance depends on the age, mass, and environmental conditions of each quenched galaxy. These mechanisms can be divided into two classes based on their origins: internal (coming from within the galaxy being quenched) and environmental (coming from surrounding galaxies). Internal mechanisms, most notably active galactic nucleus (AGN) feedback, are responsible for most of the quenching seen in high-mass galaxies, while environmental mechanisms contribute to the quenching of low-mass galaxies, especially if said galaxies are satellites around a more massive central galaxy.

In large surveys of the Universe, galaxies generally display a bi-modality, with distinct populations of blue, star-forming galaxies versus red, quenched ones. To quantify this bi-modality, astronomers use ${\textstyle {sSFR}}$, the specific star formation rate of a galaxy, which can be defined simply as:

${\displaystyle sSFR={SFR \over M_{*}}}$

where ${\textstyle SFR}$ is the total star formation rate, measured in solar masses per year, and ${\textstyle M_{*}}$ is the total stellar mass, measured in solar masses. A rigid definition of quenching in the local Universe sets the quenching threshold at ${\displaystyle sSFR<10^{-11}}$.

However, in the grand scheme of cosmic history, the picture becomes more complicated. Across cosmic time, the star formation rate and stellar mass of galaxies have evolved significantly. For example, the Universe is theorized to have had elevated rates of star formation around two to three billion years after the Big Bang, a period also known as "Cosmic Noon". This is unlike our current epoch, which is called "Cosmic Twilight" as today's galaxies are forming stars at much lower rates. To accommodate for these evolving galaxy properties across different epochs, the quenching threshold has also been defined in more flexible terms. One such definition parameterizes the threshold as:

${\displaystyle sSFR={0.2 \over t_{H}(z)}}$