noun. Nautical, a platform at the masthead; a top.

noun. In heraldry, an inclosed circular platform, like a large flat tub, set upon the top of a pole, which pole is shown to be a mast by having a small yard with furled sail attached put across it, usually at an angle—the whole being a conventional representation of an ancient round top of a ship.

noun. A top; a platform at a masthead; -- so called because formerly round in shape.

noun. Alternative form of round-top.

noun. a canvas tent to house the audience at a circus performance

"From the roundtop-mast of an English ship, just brought in as a prize, he one day witnessed the steaming in of Robert Fulton's steamboat."

canvas  canvas tent  canvass  

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