Variable definition

Kotlin has two types of variables - read-only and variables with mutable values. Read-only variables are defined with the keyword "val" when mutable variables are defined with the keyword "val".

To define a variable, you have to provide a keyword identifying the variable as read-only or mutable, then specify the variable's type and then the variable's name.

val name: String = "John"

var age: Int = 23

If you define the variable's value in the expression, you can omit the type because the compiler will recognize the type from the assigned value.

val name = "John"

var age = 23

Constants

In Kotlin, constants are defined with the keywords "const val".

const val systemName = "My Super System"

Because you specify the value for constant, you can omit the type. 

It seems we have a read-only variable, so what is the difference between a read-only variable and a constant?

The main difference is that constant value should be defined at compile time. Because of this, the constant can't be assigned a function call or constructor.