an antiparticle

For every kind of matter particle, nature keeps a mirror twin: same mass, opposite charge. The electron has the positron, the proton has the antiproton, the quark has the antiquark. When a particle meets its antiparticle they can annihilate, their entire mass converting into a burst of radiation by E = m c^2. Antimatter sounds like science fiction, yet a banana emits a positron every hour or so from the decay of its potassium, and hospitals image the brain with it every day in PET scans.

An antiparticle has the same mass and spin as its particle but the opposite sign of every additive quantum number, electric charge, but also color charge, lepton number, baryon number and so on. Antiparticles were not discovered by accident but predicted: in 1928 Dirac's relativistic wave equation for the electron had solutions with negative energy, which he reinterpreted as a positively charged partner, the positron, found by Anderson in cosmic rays in 1932. In the modern view a particle and its antiparticle are two excitations of the same underlying quantum field. The deep guarantee is the CPT theorem: any sensible relativistic quantum field theory forces a particle and its antiparticle to have exactly equal mass and lifetime.

Some strictly neutral particles are their own antiparticle, the photon is the standard example, while whether the neutrino is its own antiparticle (a Majorana particle) is a major open question. Antimatter poses one of the great puzzles of cosmology: the laws of physics look almost perfectly symmetric between matter and antimatter, yet the universe is made of matter with essentially no antimatter left over. Explaining that imbalance requires CP violation and the still-incomplete story of baryogenesis. A common misconception is that antimatter has negative mass or 'falls up'; it has ordinary positive mass and, as far as experiments can tell, falls down like everything else.