In math, a radian is the angle created when the arc length of a circle is equal to the radius of that circle. If you have a slice of pizza that's as long as the length of its crust, the angle created by the pointy end of the pizza slice is a radian.

Imagine taking the radius of a circle — the line going from the center to the perimeter — and wrapping it around the outside. The angle this creates is a radian. You'll need to understand radians to study advanced math like trigonometry or calculus. The concept of a radian as a unit of measure was developed in 1714, but the word wasn't coined until the nineteenth century, from radius.

Definitions of radian
  1. noun
    the unit of plane angle adopted under the Systeme International d'Unites; equal to the angle at the center of a circle subtended by an arc equal in length to the radius (approximately 57.295 degrees)
    synonyms: rad
    see moresee less
    type of:
    angular unit
    a unit of measurement for angles
Word Family

Test prep from the experts

Boost your test score with programs developed by’s experts.

  • Proven methods: Learn faster, remember longer with our scientific approach.
  • Personalized plan: We customize your experience to maximize your learning.
  • Strategic studying: Focus on the words that are most crucial for success.


  • Number of words: 500+
  • Duration: 8 weeks or less
  • Time: 1 hour / week


  • Number of words: 500+
  • Duration: 10 weeks or less
  • Time: 1 hour / week


  • Number of words: 700+
  • Duration: 10 weeks
  • Time: 1 hour / week