A bounded volume held by surface alone. Fragile, self-closing, shaped by pressure from both sides.
English bubble (14th century), probably imitative — the word sounds like the thing, as in Dutch bobbel and German Blase. Related to Latin bulla (a bubble, knob, rounded swelling), which also gives English BULLETIN and PAPAL BULL, by way of the round lead seal on a document. A bubble has meant "something showy that bursts" since the 17th century, and the South Sea Bubble of 1720 fixed that sense.
The game realm's own word, and its anti-compulsion identity: a bubble is a small delight that is COMPLETE when it pops. Nothing is accumulated, nothing is owed, and the burst is the whole point rather than a loss.