The term modified base refers to
A modified base is one that has been modified after nucleic acid chain has been formed. These bases are modified by methylation, reduction, acetylation or glycosylation.
In t-RNA there are many modified bases, e.g. dihydrouridine, ribothymidine, pseudouridine, and 7-methylguanosine.
In DNA, the most common modified base is 5-methylcytosine.
Other important modified bases are acetylcytosine and dimethyladenine.
Purine or pyrimidine attached to deoxyribose
A purine or pyrimidine that has been altered
A nitrogen containing ring other than a purine or pyrimidine that is part of a nucleotide
A purine or pyrimidine attached to a sugar by O-glycoside linkage