spontaneous
spon-TAY-nee-usadjMeaning happening naturally, without planning
Memory hook SPONT-aneous: it just HAPPENS. One neighbor starts strumming a guitar on her steps, and soon the whole street is dancing, with no plan at all. The party is spontaneous.
The crowd broke into spontaneous applause when the puppy crossed the finish line.
In context Nothing about the party was planned: it was spontaneous, starting when one neighbor brought out a guitar and others drifted over with snacks and folding chairs.
Similar unplanned, impromptu, natural, unrehearsed
Opposite planned, rehearsed, deliberate
From Latin sponte, “of one’s own accord, freely.”