A taxonomic order is a taxonomic rank below a taxonomic class and above a taxonomic family. An order is a named grouping, or taxon, at that position in a classification. Its constituent families can themselves contain several genera and species.[ref]
Biology 2e by Mary Ann Clark and others. section 20.1, Classification Levels
For example, Carnivora is an order containing the cat family Felidae. The order name and family name identify groups at different levels; the same animal belongs to both. This is nested classification rather than two competing identifications.[ref]
A constructed database example illustrates the information lost during aggregation. Suppose an order contains three families with 2, 4 and 1 sampled species respectively. Reporting seven species for the order preserves the total but discards how those records were distributed among families. An analysis at order level can therefore conceal variation visible at family level.
The rank specifies hierarchical placement, not a fixed age, number of species or amount of genetic divergence. A phylogenetic tree instead expresses a hypothesis about ancestry and branching. Classification uses evolutionary evidence, but a rank label alone is not a numerical evolutionary distance.[ref]
Biology 2e by Mary Ann Clark and others. section 20.1, phylogenetic hypotheses and limitations of tree diagrams
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