Biostratigraphic unit or biozones are intervals of geological strata that are defined on the basis of their characteristic fossil taxa.
A biostratigraphic unit may be defined on the basis of a single taxon or combinations of taxa, on relative abundances of taxa, or variations in features related to the distribution of fossils. The same strata may be zoned differently depending on the diagnostic criteria or fossil group chosen, so there may be several, sometimes overlapping, biostratigraphic units in the same interval.
The length of time represented by a biostratigraphic zone is called a biochron.
References
Biozone Wikipedia(Text) CC BY-SA