If something is undefined it's not clear, such as your undefined vacation plans. You know you want to go somewhere during the third week of August, but you just don't know where.

You can tell just by looking at it that undefined means "not defined." But you might be surprised that the root define comes from the Old French word defenir, meaning "to end, terminate, determine." In this case, define carries the meaning of "determine," but in a sense "end" and "terminate" apply as well. When you define something, that's the end of it — no more wondering. So if something is undefined, it's not yet determined.

Definitions of undefined
  1. adjective
    not precisely limited, determined, or distinguished
    “an undefined term”
    undefined authority”
    “some undefined sense of excitement”
    synonyms: vague
    indefinable, undefinable
    not capable of being precisely or readily described; not easily put into words
    vague or not clearly defined or stated
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    clearly characterized or delimited
    precise; explicit and clearly defined
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