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In “taxicab geometry”, circles are square-shaped. Wait … circles and squares are different shapes. Yes, this is true, but mathematically, a circle does not have to be circle-shaped. To see this, we need look at the definition of a circle and consider unusual geometries. There are other geometries? Yup!

In all geometries, the definition of a circle is the same: a circle is the collection of all points some fixed distance from a fixed center. In taxicab geometry, distance is strange (above left) so circles are too. A taxi can only drive on streets so distance from one point to another is calculated considering the shortest possible path to take along the streets not necessarily the straight line distance from point A to point B. In taxicab geometry, the only points that exist are points on a grid and 1 unit of distance is one city block.

On the right are examples of circles in taxicab geometry. The top one, for example, represents all the points 2 blocks away from the center blue point (and hence a circle of radius 2). As we can see these circles end up being square-shaped!

All this may seem like just fun and games but these ideas have a deep mathematical significance. In the area of mathematical analysis, taxicab geometry has connections to L^p spaces.

For more info see https://en.wikipedia.org/wiki/Taxicab_geometry