enzymes that cut internal bonds of a polymer. Endonucleases are able to cleave phosphodiester bonds within a nucleic acid chain by hydrolysis either randomly or at specific base sequences (cf. restriction enzymes).
Source:
PAC, 1992, 64, 143. 'Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)' on page 152 (https://doi.org/10.1351/pac199264010143)