open sentence
noun
: a statement (as in mathematics) that contains at least one blank or unknown and that becomes true or false when the blank is filled or a quantity is substituted for the unknown
Example Sentences
Recent Examples on the Web Firstly, that opening sentence caught a lot of people’s attention. 
Graeme Mcmillan, WIRED, 8 July 2018 Rossetti said the plea deal was altered to involve an open sentence by the judge upon the guilty plea, but officials said Sherman, a Fox Lake resident, could not be sentenced to more than five years under the current charges against her. Jim Newton, Lake County News-Sun, 7 Sep. 2017
Graeme Mcmillan, WIRED, 8 July 2018 Rossetti said the plea deal was altered to involve an open sentence by the judge upon the guilty plea, but officials said Sherman, a Fox Lake resident, could not be sentenced to more than five years under the current charges against her. Jim Newton, Lake County News-Sun, 7 Sep. 2017 Word History
First Known Use
1937, in the meaning defined above