Two sets are called disjoint if they have no elements in common.

e. g. The set of even integers and the set of odd integers are disjoint sets as every integer is either odd or even but not both.  

Numbers

Square root of a number . . . 

Consider the equation 

x^2 -25=0, the solutions are 5&-5. Now consider ,  the equation x^2 +25=0. The solutions of this equation are square root of -25 which are not real numbers. As square root of a negative number is not real.  It is called as imaginary number. Square root of -1 is imaginary and is denoted by i .  The set of complex numbers is denoted by C and is of great importance in mathematics.