Group of plants with similar characteristics of any rank is termed

A Group of plants with similar characteristics of any rank is termed a taxon.
A taxon is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit.
A taxonomic unit, whether named or not: i.e. a population, or group of populations of organisms that are usually inferred to be phylogenetically related and which have characters in common that differentiate.