A hierarchy is an ordering of persons, ideas, or constructs from less to greater importance, in a tiered organizational structure. Organizations may have a hierarchy, both formally and informally: for example, the Senate formally has no internal hierarchy, but informally, senior Senators may "rank" above junior Senators. Doctors of academic disciplines (Ph.D.) in prestigious universities who have been awarded the distinction of occupying the "chair" of academic study are sometimes regarded as "high priests" of their discipline, and the varying levels of academic expertise in Boards of Education as representative of a hierarchy of education. Head coaches of sports teams and their staffs of assistants each constitute a hierarchy of athletic expertise. Military rank is an explicit form of operational hierarchy in every nation, for example, in the United States, from the Commander in Chief, the President, and the Joint Chiefs of Staff, down to the least member of enlisted military personnel. The CEO of a corporation represents the head of a hierarchy of business in the world of economics.
The word hierarchy comes from hieros, "priest", and archos, "ruler", thus "priest-rule", from the ancient societal ordered levels of authoritative rule by a form of highly educated "priesthood", either secular, as in politics, education and science, or religious, either pagan or Christian. The most readily recognized hierarchy is exemplified by the levels of religious authority in the Catholic Church.
Intellectually, ideas may be expressed in a hierarchical form. An idea common in philosophy is the "hierarchy of needs," a theory proposed by Abraham Maslow, who argued that each individual must meet different needs in ascending order before attaining self-actualization, the pinnacle of human self-awareness.
Taxonomy refers to a generic structure or classification system into which items may be classified based on units referred to as taxa (singular taxon). It often takes the form of a hierarchical ordering of taxa, as expressed in Linnaean taxonomy.
The Bible reveals the existence of a multitude of spirit-beings, and even the existence of a spiritual hierarchy. According to the Jewish Kabbalah and other forms of Gnosticism, existence is ordered according to a mystical hierarchy of being emanating from one source of intelligent power. The religion of Hinduism states that there is only one God which has no form and is infinite, which Hindus call Paratman (Supreme Soul), Brahmana (Eternal Spirit), or Ishvara (Supreme Lord), that manifests in many forms as a hierarchy of Deities that have various names but represent aspects of God that manifest in the world.