Angels is one of the nine choirs, or orders, of angels described in The Celestial Hierarchy, a treatise written around the late fifth or early sixth century by the anonymous Christian author known as Pseudo-Dionysius the Areopagite. That work arranges the angels in three triads: the highest of Seraphim, Cherubim and Thrones; the middle of Dominions, Virtues and Powers; and the lowest of Principalities, Archangels and Angels. The scheme drew together the various ranks named in scripture, the seraphim of Isaiah, the cherubim of Genesis and Ezekiel, and the thrones, dominions, principalities and powers listed in the Pauline epistles, into a single ordered hierarchy that became standard in medieval Christian theology and was carried further by Thomas Aquinas and by Dante. This entry describes Angels as a figure of that documented theological and literary tradition.
The Greek angelos simply means "messenger" and, in its most general sense, names the whole created order of these beings rather than any single rank; Pseudo-Dionysius narrows the common word to the ninth and lowest choir specifically, the rank he holds to stand closest to humanity and most directly engaged with individual human affairs, including, in later tradition though not explicitly in his own text, the office of personal guardian angels assigned to particular people rather than to nations or nature.