Comment by SAI_Peregrinus

A block is just the distance from one intersection to the next. Even if those distances vary or are non-square.

E.g. Manhattan has mostly rectangular blocks, if you go from 8th Avenue to Madison Avenue along 39th St you traveled 4 blocks (the last of which is shorter than the first 3), if you go from 36th St to 40th St along 8th Avenue you traveled 4 blocks (all of which are shorter than the blocks between the avenues).