If M is a metric space, and d > 0 is a real number, then the d-dimensional Hausdorff measure Hd(M) is defined to be the infimum? of all m > 0 such that for all r > 0, M can be covered by countably many closed sets of diameter < r and the sum or the d-th powers of these diameters is less than or equal to m.
The Hausdorff dimension d(M) is then defined to be the infimum of all d > 0 such that Hd(M) = 0. The Hausdorff dimension is well-defined for any metric space M and we always have 0 ≤ d(M) ≤ ∞.