From Middle English undecided, equivalent to un- + decided.

undecided

1. Open and not yet settled or determined.

   Synonyms: indeterminate, unresolved, unsettled, indecisive

   The question of man's destiny is still undecided.

2. Uncommitted, not having reached a decision.

   Synonyms: ambivalent, irresolute, of two minds, indecisive, unsure

   The result of the election is in doubt because of a large number of undecided voters.

• German: unbestimmt, unbeantwortet, unschlüssig, unentschieden
• Portuguese: indecidido
• Russian: нерешённый

• German: unentschlossen, unentschieden
• Portuguese: indeciso
• Spanish: indeciso

undecided (plural undecideds)

1. A voter etc. who has not yet come to a decision.
   • 2016, Herb Asher, Polling and the Public: What Every Citizen Should Know:

     […] but that is a highly flawed procedure if the undecideds differ in major ways from the decideds.

1. Simple past tense and past participle of undecide