Definitions of the main concepts of plate-tectonics ( taken from Allan Cox and Robert Brian Hart's 'PLATE TECTONICS: How It Works' - Blackwell Scientific Publications ) and some of the relevant concepts of geophysics ( taken from Apple Computer's 'Dictionary and Thesaurus', Version 1.0.1 )

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LITHOSPHERE

Rigid outer layer of the Earth

ASTHENOSPHERE

Fluid layer beneath the lithosphere.

PLATE

Non-deformable block of lithosphere with a perimeter consisting of boundaries of the following three types.

RIDGE ( also: extensional boundary / fault )

Boundary where two plates are diverging. Along the opening crack, magma rises from the asthenosphere and solidifies on both diverging plates. Ridges are symmetrical in the sense that the two plates usually grow at the rate. Relative plate motion across a ridge is not necessarily perpendicular to the ridge.

TRENCH ( also: compressional boundary / fault )
Boundary where two plates are converging. One plate moves beneath the other, eventually to be absorbed into the mantle. Trenches are always asymmetrical in the sense that one plate is underthrust, and its leading edge is 'destroyed', whereas the other plate is not shortened. Relative motion across a trench is generally not perpendicular to the trench.

TRANSFORM ( also: sliding boundary / fault )
Boundary along which plate motion is exactly parallel to the boundary. Lithosphere is neither created nor destroyed along a transform. Geometrically, transforms are always circles concentric about the Euler pole for the two plates. In the limiting case of a pole at infinite distance on a plane, the transform is a straight line.

SUBDUCTION

The sideways and downward movement of the edge of a plate of the earth's crust into the mantle beneath another plate.

EULER POLE
The pivot point about which two plates rotate relative to each other. The Euler pole is the only point that does not move relative to either plate. The Euler pole for two plates may be found by constructing perpendiculars to local segments of their transform faults.

FRACTURE ZONE
Narrow submarine mountain range marking the location of a present or past transform. Both active and inactive fracture zones are concentric about the Euler pole.

PRE-CAMBRIAN Of, relating to, or denoting the earliest eon, preceding the Cambrian period and the Phanerozoic eon - the Precambrian eon or the system of rocks deposited during it. The Precambrian extended from the origin of the earth (believed to have been about 4,600 million years ago) to about 570 million years ago, representing nearly ninety percent of geological time. The oldest known Precambrian rocks have been dated to about 3,800 million years old, and the earliest living organisms date from the latter part of the eon. The Precambrian is now replaced in formal stratigraphic schemes by the Archean, Proterozoic, and (in some schemes) Priscoan eons. PALEOMAGNETISM Tthe branch of geophysics concerned with the magnetism in rocks that was induced by the earth's magnetic field at the time of their formation.