Definition interval: just an interval, representing the interval defined by a function, regardless of the endpoint.

Domain: it is the range of all the independent variables that make the function meaningful, and the endpoints should also be taken into account.

Give two examples:

(1) f (x) = x 2 The domain is r or (-∞, +∞).

The defined interval is (-∞,+∞)

(2) f (x) = sqrt (-x 2) means that the square of the radical sign is negative x.

The domain is x=0.

It has no defined interval.

That is to say, when the domain is a constant or several discontinuous constants, there is no definition interval. For others, it can be considered that the defined interval is the defined domain.