magnitude: (see also vector and scalar; for an example, compare velocity, and speed)
mechanics, quantum: (see above, also off-site: Rudiments of Quantum Theory)
mechanics, statistical: describes the mechanics (motions and interactions) of many particles via statistical analysis. In other words, instead of considering the motions of every particle individually, statistics allows the consideration of how particles act on average and give meaningful properties of the collection of particles rather than information about what is happening to any individual particle.
meson: These particles are not fundamental. These are made up of quarks and anti-quarks. Mesons are a sub-category of hadrons; that is to say, all mesons are hadrons, but not all hadrons are mesons. A meson is made up of a constituent quark and anti-quark pair. These need not be the same flavor. However, they must have opposite color charge, leaving the meson uncharged with respect to the strong interaction. It could still be electrically or otherwise charged.
momentum, angular: (see momentum)