From WordNet (r) 3.0 (2006) :
open interval
n 1: an interval that does not include its endpoints [syn: open
interval, unbounded interval] [ant: bounded interval,
closed interval]
From The Free On-line Dictionary of Computing (30 December 2018) :
open interval
A type of interval (range of numbers) that does
not include either of its endpoints.
For example, when mixing red and blue paint, the proportion of red
lies in the interval 0% to 100% but can't be exactly 0% or 100% or
it wouldn't be a mixture.
(2015-04-26)