While each city is different from another we can define a block as any an are in a metropolitan bounded by four streets roads or the length of one side of a land parcel or gathering of structures, from where one road crosses it to the following. Every city is distinctive with various estimated blocks and diverse measured parcels for homes or business. So whenever you are crossing one street you are leaving one block and entering another block.
It can be square or rectangular shape. When say a house is 3 blocks away, it means that you have to three streets to reach the block where its located. Blocks can be long or short. It would take you around 3 minutes to walk one square.
In countries like the US, Canada and Australia, a block in some instances is used as a unit of measuring. In Manhattan, USA a square is 264 x 900 feet ( 80 x 274 meters) , in Edmonton, Canada a square is 330 x 560 feet ( 100 x 170 meters ) ref. wikipedia. The good ways starting with one convergence then onto the next in these models fluctuates from 264 to 900 feet (80 to 274 meters) .