Metallic glasses, quasicrystals and many intermetallic compounds belong to complex alloy phases which are characterized by specific nearest-neighbor coordination polyhedral clusters representing the local atomic features of their parent phases. However, the nearest neighbor atoms are often distributed on multiple shells, which makes the cluster difficult to define exactly. For an alloy phase dominated by the clusters, there showed appear distinct differences between the cluster part and the average structure of the phase, especially with regards to the atomic density. The clusters should have the highest atomic density so that their presence is very pronounced in phase compared with the average structure. In this paper, radial distribution of atomic density (numbers of atoms contained in different spherical shells) is proposed to define the cluster in a clear manner. The spherical shell enclosing the highest atomic density and having dense-packed triangular facets is selected as the nearest-neighbor cluster. Al-Ni-Zr alloy phases are taken for example to illustrate the easy use of this method, and the relationship between cluster and metallic glass formation is also explained.