Qualifying a solemn papal blessing, proclamation, etc., the post-classical Latin adverb urbi et orbi means to the city (of Rome) and to the world.

It is from classical Latin urbī, dative of urbs, city, and orbī, dative of orbis, orb, circle. In classical Latin, orbis terrarum, also orbis terrae, literally the orb, also the circle, of the earth, meant by extension the world, since the ancients regarded the earth as a circular plane or disk.

The words urbs and orbis were already used in collocation in classical Latin; for example, in Fasti, a poem on the Roman festivals, the Roman poet Ovid (Publius Ovidius Naso – 43 BC-circa 17 AD) wrote:

gentibus est aliis tellus data limite certo:
Romanae spatium est urbis et orbis idem.
     translation:
The lands of other nations have fixed boundaries:
The extent of the City of Rome and of the world is one.

Theodulf of Orléans (circa 750-821), poet, theologian and Bishop of Orléans, applied the collocation to Pope Adrian I, who died in 795, in the epitaph titled Super Sepulchrum Hadriani Papae:

Tu decus Ecclesiae, fax splendens urbis et orbis.
You glory of the Church, blazing torch of the city and of the world.

The form urbi et orbi occurs around 1276 in Cæremoniale Romanum editum jussu Gregorii X, the ritual for the investiture of a new Pope:

investio te de Papatu Romano, ut praesis urbi et orbi.
I invest you in the Roman papacy and you preside over the city and over the world.

The genitive urbis et orbis came to be prefixed to official documents. The first of these, dating from 1670, is titled Extensio concessinis [sic] officij, & missae B. Rosae de Sancta Maria Tertij Ordinis S. Dominici pro vniuerso clero Regnorum Poloniae, Ducatu Lituaniae &c.:

Urbis, et orbis. Concessio ut in toto orbe terrarum celebrari possit Missa B. Rosae ex voto, vel devotione.
Of the city, and of the world. A concession that the mass of the Blessed Rose may be celebrated in the whole world by prayer, or devotion.

The first known use of urbi et orbi in an English text is from An Essay on the Law of Celibacy imposed on the Clergy of the Roman Catholic Church (Worcester, 1781), by John Hawkins (1719-89), English lawyer and music scholar:

     from Encyclopædia Londinensis; or, Universal Dictionary of Arts, Sciences, and Literature (London, 1817)
We cannot observe, without a mixture of surprise and indignation, that in this enlightened age, many legends equally absurd are still retained in the Breviaries or Common Prayer Books actually used by the clergy in Roman-catholic countries. That we may not be accused of advancing this without sufficient warrant, we shall select a few examples, from many others equally exceptionable and romantic. […] St. Bonaventure relates, that a crucified seraphim, fastened to a cross in the same manner as our blessed Saviour was, appeared upon a certain occasion to St. Francis, and discoursed some time with him in a familiar manner. When the angel vanished out of sight, the saint found himself marked with an impression of our Saviour’s wounds in his hands and feet, the heads of the nails appearing on one side, and the points on the other. In his breast he had a red scar or wound, which often pouring forth his sacred blood covered his drawers and tunic with stains. He calls this rem admirabilem, “an extraordinary wonder,” et tantopere testatum, “yet fully attested.” A feast instituted in remembrance of this miracle is held forth urbi et orbi, that is, ordered to be kept in the divine office over the whole Catholic church.

In extended use, the adverb urbi et orbi means to everyone and for general information or acceptance. In Education Reform; or, The Necessity of a National System of Education (London, 1836), the Irish politician and diplomat Thomas Wyse (1791-1862) wrote that providing public libraries

has been particularly attended to in Germany. An Englishman entering the library at Göttingen finds a better collection of English history and statistics than he usually meets with, in most libraries at home. Their universities are worthy of their libraries; they are truly, and in every sense, “Urbi et Orbi.”