"Transient" pertains to things in a state of transition, like "transitory," but is slightly different. It's said of things that are both of short duration (like 'temporary') and are likely to re-occur. For example, mail servers often talk about "Transient delivery errors," when mail can, temporarily, not be delivered (errors that come and go, as it were, like DNS errors).*
"Transient" can also be said of people in transit, like boarders and such.
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* We speak of 'transient' errors in this case, because 'temporary' just indicates errors that are of short duration, whereas 'transient' errors has the added meaning of recurrence: like errors that pop up a while, go away, then briefly re-appear again, etc.