27 (twenty-seven) is the natural number following 26 and preceding 28.

Many talented and famous rock/blues musicians died at age 27. These include Robert Johnson, Brian Jones, Jimi Hendrix, Janis Joplin, Jim Morrison, Ron McKernan, Kurt Cobain, and Amy Winehouse. The musicians who died at this age are often referred to as the 27 Club.

There are many songs titled just "Twenty-Seven," so it will suffice to list the most famous: a song by Scottish band Biffy Clyro from their 2002 album, Blackened Sky; the song by the Dave Matthews Band "#27," which they began playing on their 2007 summer tour; the Lagwagon song on their album Double Plaidinum. A song by rock band Fall Out Boy also bears the title "27," and is on their 2008 album Folie a Deux.

An Atlanta based pop/punk band named Cartel also has a song titled "27 Steps" in their 2009 album Cycles.

The number also occurs buried in the lyrics without occurring in the title. American parodist "Weird Al" Yankovic hides the number in many of his songs and videos.Ben Weasel also likes to include the number in his songs for his bands Screeching Weasel and The Riverdales.

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).