The Collaborative International Dictionary of English v.0.48:

Columnar \Co*lum"nar\, a. [L. columnaris, fr. columna.]
   Formed in columns; having the form of a column or columns;
   like the shaft of a column.
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   Columnar epithelium (Anat.), epithelium in which the cells
      are prismatic in form, and set upright on the surface they
      cover.

   Columnar structure (Geol.), a structure consisting of more
      or less regular columns, usually six-sided, but sometimes
      with eight or more sides. The columns are often fractured
      transversely, with a cup joint, showing a concave surface
      above. This structure is characteristic of certain igneous
      rocks, as basalt, and is due to contraction in cooling.
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The Collaborative International Dictionary of English v.0.48:

Structure \Struc"ture\, n. [L. structura, from struere,
   structum, to arrange, build, construct; perhaps akin to E.
   strew: cf. F. structure. Cf. Construe, Destroy,
   Instrument, Obstruct.]
   1. The act of building; the practice of erecting buildings;
      construction. [R.]
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            His son builds on, and never is content
            Till the last farthing is in structure spent. --J.
                                                  Dryden, Jr.
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   2. Manner of building; form; make; construction.
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            Want of insight into the structure and constitution
            of the terraqueous globe.             --Woodward.
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   3. Arrangement of parts, of organs, or of constituent
      particles, in a substance or body; as, the structure of a
      rock or a mineral; the structure of a sentence.
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            It [basalt] has often a prismatic structure. --Dana.
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   4. (Biol.) Manner of organization; the arrangement of the
      different tissues or parts of animal and vegetable
      organisms; as, organic structure, or the structure of
      animals and plants; cellular structure.
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   5. That which is built; a building; esp., a building of some
      size or magnificence; an edifice.
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            There stands a structure of majestic frame. --Pope.
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   Columnar structure. See under Columnar.
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