Two (or more) ions are isobaric if they have the same nominal mass but different exact mass. For example positive radical ions of N2, CO, and C2H4 all have nominal mass of 28 units (z = 1). However, their exact (monoisotopic) masses are 28.00559, 27.99436, and 28.03075 units respectively.
TMT reagents are called isobaric tags, which can be confusing. These labeling reagents within each set (e.g. 2-plex or 6-plex) have the same nominal mass and the same exact mass, therefore they are not isobars as defined by IUPAC.