Gamma particles
Gamma particles are high energy electromagnetic radiation. They are produced by sub-atomic particle interactions, such as electron-positron annihilation, radioactive decay or inverse Compton scattering in astrophysical processes.
Gamma rays typically have frequencies above 1019 Hz and therefore energies above 100 keV and wavelength less than 10 picometers.
Hard X-rays can have higher energy than low energy gamma rays, since the distinction between the X- and gamma rays is somewhat arbitrary and depends on the context.
As a form of ionizing radiation, gamma rays can cause serious damage when absorbed by living tissue, and they are therefore a health hazard.