Q.Every unit of classification regardless of its rank is

A taxon (plural taxa) is the scientific term for a category used in classification, applicable at any level of the taxonomic hierarchy — kingdom, phylum, class, order, family, genus or species alike. Rather than naming one specific rank the way "species" or "genus" does, "taxon" is the general, rank-independent word for any such group: Kingdom Animalia is a taxon, and so is the species Homo sapiens, even though they sit at completely different levels of the hierarchy.

Every taxon is defined by a set of shared, observable characters that its members hold in common, and it is this practice of grouping organisms into taxa — based on easily observable characters — that the text calls classification. Because every rank in the hierarchy, without exception, represents a taxon, the concept is foundational to the whole system: understanding "taxon" as the general unit of classification is what makes it possible to then learn the seven specific major ranks (kingdom through species) as particular, named kinds of taxa, each nested within the one above it.

A useful way to avoid confusing taxon with species is to remember that species is only one specific rank of taxon — the narrowest one — while taxon itself is the umbrella term that also covers every broader rank, from genus all the way up to kingdom and domain.