The Theory of a Flat Universe?

If the observable universe extends in all directions (X, Y and Z) at all times, then what is the logic behind the theory that the Universe is Flat?

• I understand that by the word 'Flat', physicts mean, it is the shape of the Universe at the Macro Level. But at the Micro Level(Galaxy Level), its a minimum of 3Dimensions of space and one for Time. So, if the Universe is 'Flat' theory is proved wrong at the Micro Level, why should we still use it for the Macro Level.i.e to define the shape of the universe.

• Answer:

  The "flat" referred to while describing the shape of Universe does not mean Cardboard/paper type flat, rather it is used as a analogy with the way the earth's shape was perceived in the olden times, The flat earth theory said that if you traveled sufficiently far you will eventually come to the edge/end of the earth and fall if you take steps beyond that, In later times of course it was discovered that you will just end up coming back to the point you started from. So by a flat universe what is meant is that if you somehow travel sufficiently far you will come to some sort of edge/end/whatever, In contrast in a round universe you will come back to the same point from where you started.