Imperial. Blow a stream of bubbles! What is a bubble? 1. A thin, usually spherical or hemispherical film of liquid filled with air or gas: a soap bubble. 2. A globular body of air or gas formed within a liquid: air bubbles rising to the surface. 3. A pocket formed in a solid by air or gas that is trapped, as during cooling or hardening. 4. A. The act or process of forming bubbles. B. A sound made by or as if by the forming and bursting of bubbles. The Toy Association.