The article presents problems that arise when we try to define cluster concepts by listing their necessary and sufficient features. This definition does not work because for cluster concepts none of the features is necessary, but all together the features are sufficient. The article considers a very original way of defining cluster concepts developed by David Cooper, called sufficiency definition. Although this approach is disputable, it is interesting in the sense that it allows a better understanding of the nature of cluster concepts. Cooper proposed to apply to them the definition of the form: "A =_dfh Something having .sufficient of the properties P₁, P₂, P_n". The main difference between such definitions and traditional ones is that they contain an indication of a sufficient number of properties. The reference to a sufficient number of properties is justified by the fact that in relation to cluster concepts it is impossible to list necessary conditions for their