The pairing of complementary nucleotide bases (adenine and thymine, guanine and cytosine) to each other via hydrogen bonds from opposite strands of a double stranded nucleic acid (such as DNA or RNA), thereby holding the double-stranded nucleic acid together.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.