In mathematics

• Because 90 is the sum of its unitary divisors (excluding itself), it is a unitary perfect number
  Unitary perfect number
  A unitary perfect number is an integer which is the sum of its positive proper unitary divisors, not including the number itself. Some perfect numbers are not unitary perfect numbers, and some unitary perfect numbers are not regular perfect numbers.Thus, 60 is a unitary perfect...
  
  , and because it is equal to the sum of a subset of its divisors, it is a semiperfect number
  Semiperfect number
  In number theory, a semiperfect number or pseudoperfect number is a natural number n that is equal to the sum of all or some of its proper divisors. A semiperfect number that is equal to the sum of all its proper divisors is a perfect number....
  
  . 90 is a pronic number
  Pronic number
  A pronic number, oblong number, rectangular number or heteromecic number, is a number which is the product of two consecutive integers, that is, n . The n-th pronic number is twice the n-th triangular number and n more than the n-th square number...
  
  . But it is also a nontotient
  Nontotient
  In number theory, a nontotient is a positive integer n which is not in the range of Euler's totient function φ, that is, for which φ = n has no solution. In other words, n is a nontotient if there is no integer x that has exactly n coprimes below it. All odd numbers are nontotients, except 1,...
  
  . It is a Perrin number
  Perrin number
  In mathematics, the Perrin numbers are defined by the recurrence relationandThe sequence of Perrin numbers starts withThe number of different maximal independent sets in an n-vertex cycle graph is counted by the nth Perrin number.-History:...
  
  , preceded in the sequence by 39, 51, 68.
• In normal space, the interior angles of a square measure 90 degrees each. Also, in a right triangle, the angle opposing the hypotenuse measures 90 degrees, with the other two angles adding up to 90 for a total of 180
  180 (number)
  180 is the natural number following 179 and preceding 181.-In mathematics:180 is an abundant number, with its proper divisors summing up to 366. 180 is also a highly composite number, a positive integer with more divisors than any smaller positive integer...
  
   degrees. Thus, an angle measuring 90 degrees is called a right angle.
• 90 is divisible by the sum of its base 10 digits, thus it is a Harshad number
  Harshad number
  A Harshad number, or Niven number in a given number base, is an integer that is divisible by the sum of its digits when written in that base. Harshad numbers were defined by D. R. Kaprekar, a mathematician from India. The word "Harshad" comes from the Sanskrit + , meaning joy-giver. The Niven...
  
  .

In science

• Ninety is the atomic number of thorium
  Thorium
  Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
  
  , an actinide
  Actinide
  The actinide or actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.The actinide series derives its name from the group 3 element actinium...
  
  . As an atomic weight, 90 identifies an isotope
  Isotope
  Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
  
   of strontium
  Strontium
  Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...
  
  , a by-product of nuclear reactions including fallout. It contaminates milk
  Milk
  Milk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many...
  
  .
• The latitude of the North Pole
  North Pole
  The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets its surface...
  
   and the South Pole
  South Pole
  The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where the Earth's axis of rotation intersects its surface. It is the southernmost point on the surface of the Earth and lies on the opposite side of the Earth from the North Pole...
  
   are 90 degrees.

In religion

In sports

• Nike Total 90
  Nike Total 90
  Nike Total 90 is a brand of Nike sportswear and equipment first introduced in 2000, and specifically designed to be used for association football...
  
   Apparel is a brand name of football apparel and football equipment from equipment bags to goalkeeper gloves
• Major League baseball
  Major League Baseball
  Major League Baseball is the highest level of professional baseball in the United States and Canada, consisting of teams that play in the National League and the American League...
  
   bases
  Bases
  Bases may refer to:*Bases , a military style of dress adopted by the chivalry of the sixteenth century.*Business Association of Stanford Entrepreneurial Students...
  
   are 90 feet (27.4 m) away in distance
• The car number most associated with former NASCAR
  NASCAR
  The National Association for Stock Car Auto Racing is a family-owned and -operated business venture that sanctions and governs multiple auto racing sports events. It was founded by Bill France Sr. in 1947–48. As of 2009, the CEO for the company is Brian France, grandson of the late Bill France Sr...
  
   team owner Junie Donlavey
  Junie Donlavey
  W.C. "Junie" Donlavey , is a former NASCAR Sprint Cup Series car owner with a team based in Richmond. Rarely fielding a car without his familiar #90, he began fielding his team in 1950. He drove for his team at first, but soon gave way to other drivers...
  
  
• The number of minutes in a football (soccer)
  Football (soccer)
  Association football, more commonly known as football or soccer, is a sport played between two teams of eleven players with a spherical ball...
  
   match.

In Wellness Plans

• P90X
  P90X
  P90X, or Power 90 Extreme, is a home exercise system developed by Tony Horton in conjunction with Beachbody CEO Carl Daikeler, Beachbody Fitness Advisor Steve Edwards, nutritionist Carrie Wyatt, Creative Director Ned Farr, and workout video director Mason Bendewald of MEGA MACE productions...
  
  is a rigorous work out plan for 90 days with proclaimed incredible results.