The Collaborative International Dictionary of English v.0.48:

base pair \base" pair`\ (b[=a]s" p[^a]r`), n. (Biochemistry,
   Genetics)
   a unit of double-stranded DNA or RNA consisting of two
   complementary bases on opposing strands of the
   double-stranded polynucleotide, bound together by hydrogen
   bonds and other non-covalent chemical forces. The bases
   comprising the base pairs are adenine, thymine, cytidine, and
   guanine. In normal DNA, the base adenine on one strand of DNA
   pairs with thymine on the opposite strand, and cytosine on
   one strand pairs with guanine on the opposite strand. The
   term base pair usually includes the sugar (ribose or
   deoxyribose) and the phosphate bound to each base to form a
   nucleotide unit. One base pair is sometimes used as a unit of
   length or size for DNA, and in this usage is abbreviated bp;
   as, a 100-bp fragment of DNA. A length of 1000 base pairs is
   a kilobase pair or kbp.
   [PJC]

WordNet (r) 3.0 (2006):

base pair
    n 1: one of the pairs of chemical bases joined by hydrogen bonds
         that connect the complementary strands of a DNA molecule or
         of an RNA molecule that has two strands; the base pairs are
         adenine with thymine and guanine with cytosine in DNA and
         adenine with uracil and guanine with cytosine in RNA