a pair of bases consisting of the pyrimidine base of one nucleotide joined by a hydrogen bond to the complementary purine base of another nucleotide: such pairs form the links between the two strands of DNA and of double-stranded RNA
See base pair in American Heritage Dictionary 4
noun
The pair of nitrogenous bases, consisting of a purine linked by hydrogen bonds to a pyrimidine, that connects the complementary strands of DNA or of hybrid molecules joining DNA and RNA. The base pairs are adenine-thymine and guanine-cytosine in DNA, and adenine-uracil and guanine-cytosine in RNA.