non-Abelian: (see above aside) This describes an algebraic operation (multiplication, addition, division, subtraction, etc.). If the operation is non-Abelian, then the elements do not commute. Although regular multiplication commutes (3x4 = 4x3 = 12) and addition also commutes (3+4 = 4+3 = 7), there are mathematical elements (matrices) which do not commute under multiplication. (If A and B are matrices, then A times B does not equal B times A.) Operations which are invariant (unchanging) when the elements are exchanged (operations which do commute) are called Abelian. These ideas are named for the developer, Niels Abel.

action:

angular momentum: (see momentum, angular)

anode: When two metal plates are placed facing each other (i.e. parallel plates -- see capacitor) in an electrical circuit the positively charged plate is called the anode (the negatively charged plate is called the cathode).

anti-matter: (see also particle, anti-)

atom: The smallest piece of matter which is still distinguishable as a particular chemical element. It consists of (see models of the atom) electrons orbiting a tiny nucleus which consists of densely packed protons and neutrons.

atom, models of: After the discovery of the electron (see Thompson) but before the discovery of the nucleus (see Rutherford), the model of the atom was the plum pudding model in which the electrons were envisioned as plums in the pudding of protons. (Neutrons were not known to exist at the time.) After Rutherford's experiments (see the off-site explanation), the model became known as the nuclear atom, the precursor to the picture of today. That the positively-charged nucleus is so small (in defiance of the electrical force which should tear the nucleus apart to separate the positive charges) was one of the clues that classical mechanics was incomplete, heralding the birth of quantum mechanics. (For more information about how we "see" that the atom has a nucleus, see the off-site explanation)