Any change in the speed with which an object moves, or the direction in which it moves.
The central portion of a galaxy which gives off unusually large amounts of energy. These are thought to be powered by supermassive Black Holes.
The height of the peak of a wave, measured relative to its center. Equivalently, the depth of the trough of a wave.
Essentially, the "opposite" of a particle. Every type of matter has a corresponding antiparticle, with the same mass but opposite charge, for example. Other numbers describing the particle will be reversed for the antiparticle.