Amat, or antimatter, is matter that is composed of the counterparts of ordinary matter.
Antiparticles have the same mass as their counterpart, but the opposite charge. For example, the antimatter counterpart of an electron is a positron, which has a positive charge instead of a negative charge. The existence of antimatter, and even the production of it in tiny quantities, has been known since the late Industrial Age Old Earth. It was first manufactured on a large scale during the later Interplanetary period, when it was used to power space craft. Antimatter is produced naturally in tiny quantities in some radioactive decays, but the most effective way to create it is via gigantic amat farms.
Antimatter on its own is stable, but an anti-particle and a particle will annihilate when they meet, with almost total conversion of matter to energy, making this an extremely effective (if dangerous) power source. Beginning in the later Interplanetary period, and for much of the Interstellar period, interstellar spacecraft (and even many interplanetary ones) used proton-antiproton reactions as their primary energy source. The disadvantage of an amat-drive (amat powered ship) is that it produces a vast plume of highly charged particles (called a "torch"). For this reason amat-powered vessels are only allowed to engage their drive when at a safe distance from any habitats.