The specific association between two complementary strands of nucleic acids that results from the formation of hydrogen bonds between the base components of the nucleotides of each strand: A=T and G=C in DNA, A=U and G=C (and sometimes G=U) in RNA (the lines indicate the number of hydrogen bonds). Single-stranded nucleic acid molecules can adopt a partially double-stranded structure through intrastrand base pairing.
Sources:
PAC, 1992, 64, 143. 'Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)' on page 147 (https://doi.org/10.1351/pac199264010143)
PAC, 1993, 65, 2003. 'Glossary for chemists of terms used in toxicology (IUPAC Recommendations 1993)' on page 2017 (https://doi.org/10.1351/pac199365092003)