MathWorld is an online encyclopedia of mathematics from Wolfram Research company based on Eric Weisstein's CRC Concise Encyclopedia of Mathematics. Unlike PlanetMath, MathWorld does not use the GNU Free Documentation License. Currently, Wolfram holds the rights to MathWorld (and hosts it on its website) and every entry is identified as being “From MathWorld-A Wolfram Web Resource.”

MathWorld was taken offline by a court injunction as a result of the CRC Press lawsuit against the Wolfram Research company and its employee (and MathWorld's author) Eric Weisstein. See the collaboration The PlanetMath FAQ for more details.

MathWorld has articles on the integers 17, 42, 163, 666 and 1729. MathWorld does not use the Mathematics Subject Classification codes to organize its entries.

In the On-Line Encyclopedia of Integer Sequences, MathWorld is one of the most often referenced mathematical web sites, with more than six thousand links as of early 2008. (Compare Wikipedia with 360 and PlanetMath with just 44).

The website is at www.mathworld.wolfram.com.