An interval that does not include the end points.
Example:
The interval 0 < x < 20 is all the numbers between 0 and 20, but not 0 or 20
Can also be written (0,20)
Imagine a company only accepts a package "less than 20 kg", so 20 kg is not included, nor is 0 kg because that is not a package: so 0 < package < 20 kg
See: Closed Interval