Explore i on MathWorld

i is the symbol used to denote the principal square root of -1, also called the imaginary unit.

I is a high school-level concept that would be first encountered in a pre-calculus course covering complex numbers. It is listed in the California State Standards for Algebra II.


Imaginary Number: In mathematics, an imaginary number is multiple of the imaginary unit i (the square root of -1).
Root: For a mathematical function, a root is a set of arguments for which the function assumes the value of zero.
Square Root: A square root of x is a number r such that r*r = x.

Classroom Articles on Complex Numbers

  • Complex Conjugate
  • Complex Plane
  • Complex Number

  • Classroom Articles on Pre-Calculus (Up to High School Level)

  • Asymptote
  • Normal Vector
  • Conic Section
  • Parabola
  • Cross Product
  • Parametric Equations
  • Curve
  • Plane
  • Determinant
  • Plane Curve
  • Domain
  • Polar Coordinates
  • Dot Product
  • Range
  • e
  • Rational Function
  • Ellipse
  • Reflection
  • Exponential Function
  • Rotation
  • Function
  • Rotation Matrix
  • Hyperbola
  • Scalar
  • Inverse Function
  • Spherical Coordinates
  • Locus
  • Tangent Line
  • Logarithm
  • Translation
  • Natural Logarithm
  • Vector