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.