phos·pho·diester bond \¦fäs(ˌ)fōˌ…-\noun orphosphodiester linkage : a covalent bond in RNA or DNA that holds a polynucleotide chain together by joining a phosphate group at position 5 in the pentose sugar of one nucleotide to the hydroxyl group at position 3 in the pentose sugar of the next nucleotide