In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted i, called the imaginary unit and satisfying the equation ; every complex number can be expressed in the form , where a and b are real numbers. Because no real number satisfies the above equation, i was called an imaginary number by René Descartes. For the complex number , a is called the real part, and b is called the imaginary part. The set of complex numbers is denoted by either of the symbols or C. Despite the historical nomenclature, "imaginary" complex numbers have a mathematical existence as firm as that of the real numbers, and they are fundamental tools in the scientific description of the natural world.

गणित में समिश्र संख्याएँ वास्तविक संख्याओं का विस्तार है। किसी वास्तविक संख्या में एक काल्पनिक भाग जोड़ देने से समिश्र संख्या बनती है। समिश्र संख्या के काल्पनिक भाग के साथ i जुड़ा होता है जो निम्नलिखित सम्बन्ध को संतुष्ट करती है: