Antiparticle
Particle-antiparticle pairs can annihilate each other if they are in appropriate quantum states. They can also be produced in various processes. These processes are used in  particle accelerators to create new particles and to test theories of particle physics. High energy processes in nature can create antiparticles. These are visible in cosmic rays and in certain nuclear reactions. The word antimatter properly refers to (elementary) antiparticles, composite antiparticles made with them (such as antihydrogen) and to larger assemblies of either.