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]