What does "bubble" mean?

Additional senses

1. 2.A globule of air, or globular vacuum, in a transparent solid; as, bubbles in window glass, or in a lens.
2. 3.A small, hollow, floating bead or globe, formerly used for testing the strength of spirits.
3. 4.The globule of air in the spirit tube of a level.
4. 5.Anything that wants firmness or solidity; that which is more specious than real; a false show; a cheat or fraud; a delusive scheme; an empty project; a dishonest speculation; as, the South Sea bubble. Then a soldier . . . Seeking the bubble reputation Even in the cannon's mouth. Shak.
5. 6.A person deceived by an empty project; a gull. [Obs.] "Ganny's a cheat, and I'm a bubble." Prior.
6. 7.To rise in bubbles, as liquids when boiling or agitated; to contain bubbles. The milk that bubbled in the pail. Tennyson.
7. 8.To run with a gurdling noise, as if forming bubbles; as, a bubbling stream. Pope.
8. 9.To sing with a gurgling or warbling sound. At mine ear Bubbled the nightingale and heeded not. Tennyson.