The Collaborative International Dictionary of English v.0.48:

plate tectonics \plate tectonics\, n. (Geol.)
   A geological theory which holds that the crust of the earth
   (the lithosphere) is divided into a small number of large
   separate plates which float and move slowly around on the
   more plastic asthenosphere, breaking apart and moving away
   from each other at points where magma upwells from below,
   and, driven by such upwellings and other currents on the
   athenosphere, sliding past each other, colliding with each
   other, and in some cases being submerged (subducted) one
   below the other. This theory is now widely accepted, and
   explains many geological phenomena such as the clustered
   locations of earthquakes, mountain building, volcanism, and
   the similarities observed between the geology of continents,
   such as South America and Africa which are now far apart,
   but, according to the theory, were once joined together. The
   motions of such tectonic plates are very slow, typically only
   several centimeters per year, but over tens and hundreds of
   millions of years, cause very large changes in the relative
   positions of the continents. The consequence of such movement
   of plates is called continental drift.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

Tectonics \Tec*ton"ics\, n.
   1. The science, or the art, by which implements, vessels,
      dwellings, or other edifices, are constructed, both
      agreeably to the end for which they are designed, and in
      conformity with artistic sentiments and ideas.
      [1913 Webster]

   2. (Geol. & Phys. Geog.) the branch of geology concerned with
      the rock structures and external forms resulting from the
      deformation of the earth's crust; also, similar studies of
      other planets. Also called structural geology.
      [PJC]

   plate tectonics a geological theory which considers the
      earth's crust as divided into a number of large relatively
      rigid plates, which move relatively independently on the
      more plastic asthenosphere under the influence of magmatic
      upwellings, so as to drift apart, slide past, or collide
      with each other, causing the formation, breakup, or
      merging of continents, and causing volcanism, the building
      of mountain ranges, and the subduction of one plate
      beneath another. In recent decades a large body of data
      have accumulated to support the theory and provide some
      details of the mechanisms at work. One set of supporting
      observations consists of data showing that the continents
      have slowly moved relative to each other over long periods
      of time, a phenomenon called continental drift. Africa
      and South America, for example, have apparently moved
      apart from a connected configuration at about 2 to 3 cm
      per year over tens of millions of years.
      [PJC]

WordNet (r) 3.0 (2006):

plate tectonics
    n 1: the branch of geology studying the folding and faulting of
         the earth's crust [syn: tectonics, plate tectonics,
         plate tectonic theory]