Why is Madison Square Garden or Boston Garden called a "Garden"?

The word garden comes from the Latin 'geard' which means open space. Yard and GARDen come from this word.

Thus there all sorts of gardens, not just flower gardens...

For example: beer gardens, etc.

Madison Square refers to to the intersection that the arena is on, and garden is an open-spaced venue.

Boston Square Garden

The term "garden" in British English refers to an enclosed area of land, usually adjoining a building. Just another term for arena.

cause they used to grow championships there not anymore though mabye Boston will get one

Probably, but I'm not convinced

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