What is the opposite of a redshift in light?

The opposite of a redshift in light is a blueshift.

A blueshift is a decrease in wavelength accompanied by an increase in frequency and photon energy. In the visible portion of the spectrum, the shift is toward the blue end, which is how the term gets its name. The term also applies to invisible electromagnetic radiation, including radio, infrared, ultraviolet, and X-rays.

A Doppler blueshift occurs when a light source moves toward the observer. Astronomers use the shift in spectral lines to measure relative motion, including the motion of stars in binary systems and the rotation of galaxies. For example, the side of a rotating galaxy moving toward Earth can show a slight blueshift, while the opposite side shows a redshift.

Blueshift does not always mean that an entire object is visibly turning blue. Most astronomical shifts are too small for human eyes to notice and are measured precisely with spectroscopy. Blueshift can also arise from gravity: light falling into a gravitational well can gain energy. A famous nearby example is the Andromeda Galaxy, whose approach toward the Milky Way produces a blueshift that partly offsets broader cosmological redshift effects.