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.
   [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

columnar
    adj 1: having the form of a column; "trees with columniform
           trunks"; "columnar forms"; "a columnlike tree trunk"
           [syn: columniform, columnar, columnlike]
    2: characterized by columns; "columnar construction"