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]

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)