"Unregister" vs "Deregister"

I found this.

Deregister: to unregister

http://en.wiktionary.org/wiki/deregister

So, they seem like synonyms to me.

Here's a look at usage during the last 20 years:

As evidenced, unregister received a substantial boost shortly after the dot-com boom/bust of the late 90s, while deregister usage has been more or less the same throughout. It's worth noting that neither word is defined by any major dictionary, although some lesser dictionaries include the two. (Of course, placement in a dictionary does not mean a word is "real" or not - a word can exist anywhere - but it does indicate whether or not the word enjoys popular usage and if it has standardized spellings.)

Additionally, both the un- and de- prefixes can be defined as a reversal of action.

Despite the similarities, I'd go with popular usage and use unregister.

"Deregister" carries more of an active connotation than "unregister". For example, while the "unregistering" might involve removing your name from the guest list for a conference, "deregistering" would also include cancellation of hotel reservations, travel plans, etc.

I've checked the OALD, the NOAD and the OED and, surprisingly for me, none has "unregister".

The only similar entry is unregistered which means "not recorded".

But the OED (only this one) has deregister and gives this entry:

Deregister, v. (diːˈrɛdʒɪstə(r))
[de- + register]

trans. To remove from a register. Hence ˌderegiˈstration.

It appears also here (it's an OED online, considering the credits given), while unregister, again, is not available. I can understand that some old Dictionary doesn't have unregister, but I can't understand why the Oxford Advanced Learner's Dictionary doesn't have any entry considering it's online and therefore has more opportunities to be revised and updated. Not to mention that I've seen unregister before, because it doesn't sound new to me.

I would suggest that unregistering is the undoing of some registration that should never have taken place, e.g. a license that should never have been issued, whereas deregistering is the removal of some registration that is, perhaps, no longer valid. In the latter case, there is an element that it was once a valid registration but its status has changed. The former case suggests there should be no record the registration ever existed.

In OALD, only unregistered is found because this word is always used in passive voice. As a reference dictionary, OALD will never mention this word without -d.

And actually, grammatically, unregistered is more likely an adjective because it is a past participle, and a past participle can be often used as an adjective.

Shane's got it right, deregister is the verb to move someone from state=registered, to state=unregistered, IMHO.

With respect to defrocking/unfrocking, I think vicars are 'defrocked' by their superiors after they've been caught 'unfrocking' with the wrong people at the wrong time.

This leads to another possible distinction- One 'unregisters' themselves, and 'deregisters' others. If I am registered for notification, I can 'unregister' myself, but the notification service could deregister me if communications were lost.

Unregister means the item or person does not appear on the list because they were never on the list.

De-register means the item or person was initially listed, but has been removed and is no longer on the list.

Generally speaking, the de- and un- prefixes have different usages.

Un- more usually negates an adjective: unkind, unfair, etc. On the other hand, de- is a verb prefix, that denotes the action of removing some thing or state.

Hence: unleaded petrol/gasoline (because lead doesn't occur in petrol until it's added) but decaffeinated coffee (because the caffeine has been taken out).

However, when an adjective is from a verb, usage often slips from this, and we end up with un- prefixing a new verb. So the adjective unregistered gives rise to the verb to unregister, although deregister would be more in line with convention.

By this convention, unregistered should mean not registered (whether through active deregistration or never having been registered in the first place) and there should be no verb to unregister. Deregistered should mean actively removed from the register.

(The verb to undo is I think the most notable exception to the conventional norm.)

• prefix un- means undo(ctrl + z) of something.
  • Examples: unlike, uncover, unregister, unsubscribe,
• prefix de- means do reverse of something
  • Examples: demonetization, deactivate, decompile

I believe the most popular (and accepted) answer starts going in the right direction (thinking of what the act of "deregistering" does and the state transition), but then wrongly concludes "deregister" is redundant because the "unregistered" state is the same as what happens after "deregistering".

If you really think about the state transition after a "deregistering" action though, it is not at all the same as not having been registered in the first place. For example, consider a stock that has been "delisted" from an exchange. Depending on how it has been delisted, there may be certain actions an investor holding the stock could still do on the exchange, but of course, an unlisted stock cannot have any actions taken on it (other than to list it!).

As another set of examples, consider a licensed professional that has been "delicensed". It's interesting to note many professions have special terms for this state, e.g. a lawyer is disbarred, a priest is defrocked (there is an actual verb "frocked" but seems to have fallen into disuse). A medical doctor does seem to be "delicensed" but commonly one sees "disbar" also being used nowadays.

The point of these examples is to show that this state of being "deregistered" is hugely significant in a variety of business domains. One would not simply keep track of a boolean (true/false) but instead keep track of other variables. As the accepted answer even notes, an "uninitialized" variable means something different than a "deinitialized" variable. The former is just the default but the latter is on purpose and may signify critical business logic. It's unfortunate, and the cause of many bugs, that developers don't actually distinguish between the two cases when they should.