80 (eighty) is the natural number following 79 and preceding 81.

The sum of Euler's totient function φ(x) over the first sixteen integers is 80.

Adding up some subsets of its divisors (e.g., 1, 4, 5, 10, 20 and 40) gives 80, hence 80 is a semiperfect number.

80 is a ménage number, and is a Harshad number number in base 10.

The Pareto principle (also known as the "80-20 rule," states that, for many events, roughly 80% of the effects come from 20% of the causes.

According to Exodus 7:7, Moses was 80 years old when he initially spoke to Pharaoh on behalf of his people. Today, 80 years of age is the upper age limit for cardinals to vote in papal elections.

Jerry Rice wore the number 80 for the majority of his NFL career. The number 80 is also the retired number of Seattle Seahawks wide receiver Steve Largent; when Rice was traded to the Seahawks, Largent gave the team permission to unretire his number so Rice could wear it.

Eighty is also:

Year 80 (LXXX) was a leap year starting on Saturday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Augustus and Domitianus (or, less frequently, year 833 Ab urbe condita). The denomination 80 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.

A number is a mathematical object used to count and measure. In mathematics, the definition of number has been extended over the years to include such numbers as zero, negative numbers, rational numbers, irrational numbers, and complex numbers.

Mathematical operations are certain procedures that take one or more numbers as input and produce a number as output. Unary operations take a single input number and produce a single output number. For example, the successor operation adds one to an integer, thus the successor of 4 is 5. Binary operations take two input numbers and produce a single output number. Examples of binary operations include addition, subtraction, multiplication, division, and exponentiation. The study of numerical operations is called arithmetic.

A notational symbol that represents a number is called a numeral. In addition to their use in counting and measuring, numerals are often used for labels (telephone numbers), for ordering (serial numbers), and for codes (e.g., ISBNs).

Godfried Maria Jules Danneels (born 4 June 1933) is a Belgian cardinal of the Roman Catholic Church. He served as the Metropolitan Archbishop of Mechelen-Brussels and the chairman of the episcopal conference of his native country from 1979 to 2010. He was elevated to the cardinalate in 1983. His resignation at the age of 75 was accepted by Pope Benedict XVI on 18 January 2010.

Born in Kanegem, Tielt, West Flanders, he was the eldest of six siblings. Danneels entered the Grand Seminary of Bruges to become a priest. He was ordained on 17 August 1957 by Emiel-Jozef De Smedt, Bishop of Brugge.

He studied Thomistic philosophy at the Higher Institute of Philosophy in Leuven and theology at the Pontifical Gregorian University in Rome. After obtaining his doctorate in theology, he taught at the Bruges seminary and at the Catholic University of Leuven.

As a scholar, he carried out a profound study of the liturgy. The articles he wrote for the Dictionary of the Liturgy have made him famous throughout the Catholic world. Danneels was actively involved in writing Sacrosanctum Concilium^{[citation needed]}, a document which initiated the liturgical reform of the Second Vatican Council.

Raffaele Farina, SDB (born 24 September 1933) is an Italian Cardinal of the Roman Catholic Church. He currently serves as Archivist of the Vatican Secret Archives, Librarian of the Vatican Library, and president (Consiglio di Presidenza) of Scuola Vaticana di Paleografia, Diplomatica e Archivistica. Farina was elevated to the cardinalate in 2007.

Born in Buonalbergo, Farina entered the Congregation of St. Francis of Sales of St. John Bosco, more commonly known as the Salesians, at the Novitiate of Portici Bellavista at the age of sixteen. He professed on 25 September 1949, and made his perpetual vows on 25 September 1954; one of his sisters is also a Salesian. Farina began his studies in theology at the Pontifical Salesian Athenaeum of Turin in 1954 as well, obtaining his licentiate in theology from the Athenaeum in 1958. He was ordained to the priesthood by Bishop Michele Alberto Arduino, S.D.B., on 1 July 1958.

Farina later studied ecclesiastical history at the Pontifical Gregorian University in Rome, and received his doctorate in 1965. For the following three years, he worked on his specialization at the German Foundation "Humboldt" in Freiburg and Bonn. From 1968 to 1972, he was Professor of History of the Church (covering the origin to the fall of the Western Roman Empire in 476 A.D.) and of Methodology in the Theological Faculty of the Pontifical Salesian University; he then served as dean of the same faculty until 1974, and was rector of the university for two terms (1977–1983, 1992–1997). Farina was named regolatore of the XXI general chapter of his congregation in 1978.