non-, un-

Although both of these prefixes negate the meaning of the word they are attached to, there is a distinction in meaning between them.

Non-

The prefix non- means “other than.” In most cases, it has a neutral connotation.

• I made a non-alcoholic punch for the party.
• All of the plants in my terrarium are non-poisonous.
• Don’t talk such nonsense!

Note: Avoid using this prefix to create new words when a suitable opposite already exists:

• inaudible, not non-audible
• disagreement, not non-concurrence
• temporary, not non-permanent

Un-

The prefix un‑ means “the opposite of.”

• Check an unabridged dictionary for the definition.
• When unbuttoned, the jacket was loose.

When attached to a positive word, un‑ often gives a negative connotation.

• The man’s behaviour was unnatural.
• Her remarks were very uncharitable.

Non- vs. un-

The distinction in meaning between these two prefixes can be seen in the following pairs of examples:

• non-scientific: not connected with science (neutral)
  unscientific: lacking scientific rigour (negative)
• non-Christian: not connected with Christianity (neutral)
  un-Christian: not befitting a Christian (negative)
• non-Canadian: not connected with Canada (neutral)
  un-Canadian: not befitting a Canadian (negative)
• non-serviceable: not meant to be serviced (neutral)
  unserviceable: so worn that it can no longer be brought back into service (negative)