onetime
No longer current
Onetime (also one-time) as an adjective means former or having been something in the past: a onetime champion, the onetime capital of the empire.
It also describes something that happens only once: a onetime payment, a onetime offer.
Context usually makes clear which sense is intended—former emphasizes a past identity that no longer applies, while single occurrence emphasizes that something won't be repeated.
Previously but not now
- quondamFormer, now past
- exPrevious romantic partner
- previousOccurring before another
- pastTime already gone
bygone · preceding · antecedent · lapsed · formerly · previously · has-been · old
Likelihood of occurrence
- occasionNotable event or moment
- windowGlass-fitted wall opening
- openingGap or start
- shotQuick liquor serving
moment · break · prospect · possibility · golden · fleeting · rare · lucky
Permanently fixed in memory
- momentousOf great significance
- historicOf lasting significance
- landmarkNotable point of reference
- milestoneNotable event marking progress
watershed · red-letter · special · significant · notable · remarkable · extraordinary · exceptional
erstwhile
Previously but not now
past · bygone · preceding · antecedent · lapsed · formerly · previously · has-been · old · retired
unrepeatable
Impossible to do again
sole · lone · standalone · individual · exclusive
chance
Likelihood of occurrence
shot · moment · break · prospect · possibility · golden · fleeting · rare · lucky · serendipitous
unforgettable
Permanently fixed in memory
milestone · watershed · red-letter · special · significant · notable · remarkable · extraordinary · exceptional · life-changing