nil is a special value which means a variable has no value.  For example:

> x = 2.5

> print(x)

2.5

> x = nil

> print(x)

nil

You can test to see if a variable exists by checking whether its value is nil.

> print(x == nil)

True

 

if (x == nil){

 print("x is nil")

} else {

 print("x is not nil")

}

> x is nil

 

> x = 7

> print(x == nil)

false

> print(x)

7

nil cannot be used in any math operation. An error message will be displayed if used in a math operation.

> x = nil

> y = 7

> z = x+y

> [string "z = x+y"]:1: attempt to perform arithmetic on a nil value