We compare cluster definitions used in the framework of classical nucleation theory to estimate nucleationrates. Cluster definitions influence the magnitude of the partition functions used to calculate the equilibriumconstants that relate aggregation and fission rate constants. Therefore, cluster definitions affect calculatednucleation rates. Geometric and energetic definitions are simple to apply but are prone to systematic errors.We recommend the dynamic cluster definition as the most likely to lead to calculated rates agreeing withmeasurements. The definitions are illustrated using a simple xenon model.