why 1 is known as an unique number

Dear student, 

1 is called a unique number because it is neither a prime number nor a composite number. It has only one factor, i.e, the number itself; and to be a composite number also the number must have two factors. i.e.; 1 and the number; and a prime number also must have more than two factors.
 
Therefore, 1 is neither prime nor composite. Hence, 1 is a unique number.