- metallic-line star
- metallicity
- meteor shower
- meteor stream
- meteor trail
- meteor, shooting star
- meteorite
- meteorite crater
- meteorite shower
- meteoritics
- International Astronomical Union, IAU
- International Atomic Time, TAI
- International Earth Rotation Service, IERS
- International Latitude Service, ILS
- International Polar Motion Service, IPMS
- International Ultraviolet Explorer. IUE
- interplanetary magnetic field
- interplanetary matter
- interplanetary space
- interstellar absorption line
- interstellar cloud
- interstellar dust
- interstellar extinction
- interstellar gas
- interstellar medium
- interstellar parallax
- interstellar reddening
- interstellar scintillation
- interstellar space
- intervening galaxy
- intra-Mercurial planet
- intracluster gas
- intranetwork [magnetic] field
- intrinsic redshift
- intrinsic variable
- invariable plane
- inverse Compton effect
- invisible matter
- ionization temperature
- IRC source
- Iris
- iris photometer
- irregular galaxy
- irregular variable
- island universe
- isochrone
- isolating integral
- isophotometry
- Jacobi's integral
- Jacobus Kapteyn Telescope, JKT