In science, "cis configuration" can mean any of a couple of things:

  1. In chemistry, cis configuration indicates configuration of an organic molecule containing a double bond between two carbon atoms, where the largest, most complex R groups are on the same side:
    
            H     H
             \   /
              C=C
             /   \
          H3C     CH3
          
    
  2. Alternately, it can refer to the configuration of an organic molecule containing a ring, where the largest, most complex R groups are on the same side:
    
       H                     H     |       ___
        \                   /      |      // \\
         ring viewed edge-on       |      \___/
        /                   \      |      /---\
     H3C                     CH3   |   H3C     CH3
    
    

    These configurations are the opposite of trans configuration.

  3. In genetics, cis configuration is a term meaning an event or a gene whose action occurs on the same chromosome.

  4. Alternately, it can refer to two mutations in different genes coding for the same phenotype which are on the same chromosome (as opposed to the trans configuration where each homologous gene has one of the mutations).

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.

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