A neutrino ( new-TREE-noh; denoted by the Greek letter ν) is a fermion (an elementary particle with spin of  1 /2) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small (-ino) that it was long thought to be zero. The rest mass of the neutrino is much smaller than that of the other known elementary particles excluding massless particles. The weak force has a very short range, the gravitational interaction is extremely weak due to the very small mass of the neutrino, and neutrinos do not participate in the strong interaction. Thus, neutrinos typically pass through normal matter unimpeded and undetected.

Weak interactions create neutrinos in one of three leptonic flavors:

Each flavor is associated with the correspondingly named charged lepton. Although neutrinos were long believed to be massless, it is now known that there are three discrete neutrino masses with different tiny values (the smallest of which could even be zero), but the three masses do not uniquely correspond to the three flavors: A neutrino created with a specific flavor is a specific mixture of all three mass states (a quantum superposition). Similar to some other neutral particles, neutrinos oscillate between different flavors in flight as a consequence. For example, an electron neutrino produced in a beta decay reaction may interact in a distant detector as a muon or tau neutrino. The three mass values are not yet known as of 2022, but laboratory experiments and cosmological observations have determined the differences of their squares, an upper limit on their sum (< 2.14×10⁻³⁷ kg), and an upper limit on the mass of the electron neutrino.

For each neutrino, there also exists a corresponding antiparticle, called an antineutrino, which also has spin of  1 /2 and no electric charge. Antineutrinos are distinguished from neutrinos by having opposite-signed lepton number and weak isospin, and right-handed instead of left-handed chirality. To conserve total lepton number (in nuclear beta decay), electron neutrinos only appear together with positrons (anti-electrons) or electron-antineutrinos, whereas electron antineutrinos only appear with electrons or electron neutrinos.

Neutrinos are created by various radioactive decays; the following list is not exhaustive, but includes some of those processes:

• beta decay of atomic nuclei or hadrons
• natural nuclear reactions such as those that take place in the core of a star
• artificial nuclear reactions in nuclear reactors, nuclear bombs, or particle accelerators
• during a supernova
• during the spin-down of a neutron star
• when cosmic rays or accelerated particle beams strike atoms

The majority of neutrinos which are detected about the Earth are from nuclear reactions inside the Sun. At the surface of the Earth, the flux is about 65 billion (6.5×10¹⁰) solar neutrinos, per second per square centimeter. Neutrinos can be used for tomography of the interior of the earth.