is there any plausible way to seperate the semantics of

undefined - not defined


undetermined - not determined


  • sorry bout that, gonna edit my question
    – bngschmnd
    Nov 27, 2015 at 15:25
  • 2
    Maybe "un-" means "opposite of", while "not" is "negation of".
    – Greg Lee
    Nov 27, 2015 at 15:31
  • They mean exactly the same thing (in every instance I can think of), however the use of the "un" prefix often leads to smoother and more elegant syntax: "The police arrived to find the house unoccupied." or "He thought Jane's comments were rather unkind." The use of "un" also increases flexibility in adding qualification: "Peter was particularly unimpressed by Venice." v "Peter was not impressed by Venice particularly.", etc.
    – Cargill
    Nov 27, 2015 at 17:17
  • 1
    There could be a case for saying that "undefined" and/or "undetermined" can be used as positive or deliberate actions, as opposed to no actions or intentions occurring. So: "The exact boundaries of the estate are not defined in some sections." v "The exact boundaries of the estate are undefined so that future generations might make a further claim if the opportunity arises."
    – Cargill
    Nov 27, 2015 at 17:41
  • Uh, what is your question??
    – Hot Licks
    Nov 27, 2015 at 18:53

1 Answer 1


First of all "not" is not a prefix it is actually an adverb.

In fact prefix un is added to the front of adjectives or participles and usual meaning is opposite or not; but not is used to give the following word or phrase a negative meaning.

Not can be used with verbs, auxiliary verbs, words or phrases when the speaker is correcting them, and noun phrases with an expression of quantity.

think about these examples:

1- He is not determined to do ....

2- he is an undetermined person.

You can see the meaning is different.

  • However "determined" in these examples mean different things, rather than the meaning or sense being changed by "un" and "not". He is not determined to do ..." and "He is undetermined (as) to what to do ..." better compares like with like, it seems to me.
    – Cargill
    Nov 27, 2015 at 20:44

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