lattice field theory: (see above)
law, physical: A physical law is a relationship experimentally observed to hold in nature despite what our theories may or may not say about it. Since the goal is to describe nature, we must play by those rules which are imposed by nature. These are equivolent to mathematical postulates: that which is not derived from definitions, but rather assumed to be true so that deducible theorems can be proven. A physical law is generally unsatisfying because it describes without explaining. An example is Newton's Laws. These describe how forces cause an object to change its motion. However, it is possible (in this case) to derive these laws if a different postulate is made instead. One can either say Newton's Third Law implies Conservation of Momentum, or one can say that Conservation of Momentum implies Newton's Third "Law." These are in some sense describing the same phenomenon; it is not clear which (if either) is a "more fundamental" description.
Laws, Newton's: This set of physical laws describe how forces cause an object to change its motion. (Newton used the word "action" to describe what we now call "force"; we now have a different meaning for the word action.)
I. An object at rest or in motion will maintain its velocity unless acted upon by a force.
II. An object upon which a force acts will accelerate in proportion to its mass, such that F=ma.
III. For every force acted upon an object, there is a force of equal magnitude which acts upon the object which imposed the force but in the direction opposite to the original force. (This law must be adjusted when relativistic effects, such as the finite speed of information travel, are considered.)
lepton: This type of particle is affected by the weak interaction, but not by the strong interaction. This is to be compared against hadrons. The current theory of the standard model categorizes leptons into families.
lepton, family of: Each family of leptons has four members: a massive particle, a corresponding massive anti-particle, a massless particle, and a corresponding massless anti-particle. The families are distinguished by the mass of the massive particle. (The massive anti-particle has the same mass.) This mass is different for each family. An example of a lepton family is the set of particles: the electron, the positron, the electron neutrino, and the electron anti-neutrino. There is also a muon family of leptons and a tauon family of leptons. There could very well be more families of leptons, but these have not yet been discovered. It would be aestetically pleasing if the number of lepton families were the same as the number of quark families. Currently there are three known families of each.
light: Hertz was the first to demonstrate experimentally the production and detection of Maxwell's electromagnetic waves, showing visible light to be merely one region of colors which we happen to see. This discovery of course lead directly to radio, television, and radar, which use essentially different "colors" of light that we cannot see (i.e. which our eyes cannot process). (See also the aside)
light, speed of: The speed at which light travels (299,792,458 meters per second in a vacuum --- it slows down while traveling through material). Notice that this speed is not infinitely fast. Notice also that the fastest possible speed is numerically the same as the speed of light. This is not because it is the speed of light, but rather because light happens to be traveling at the fastest possible speed, necessarily. (I do not know why a maximum possible speed exists, but the effects of having a maximum possible speed are consistent with our universe.)