It is called definite integral because the value obtained after its integration is definite and unchangeable.

Not a function. Definite integral is a kind of integral, which is the limit of the integral sum of function f(x) in the interval [a, b]. Here we should pay attention to the relationship between definite integral and indefinite integral: definite integral, if it exists, is a concrete numerical value, indefinite integral is a function expression, and they have only one calculation relationship in mathematics.