Hillas is a term used in particle physics to describe the maximum energy a cosmic ray particle can have before it is deflected by the Earth's magnetic field. This concept was developed by John Hillas in 1984 and is used to study the origins and properties of high-energy cosmic rays. The Hillas parameter takes into account the charge and mass of the particle, as well as the strength of the magnetic field it encounters. By studying the Hillas parameter, scientists can gain insights into the processes that accelerate cosmic rays to such high energies and help unravel the mysteries of the universe.