There are unknown security risks and unexpected dangers when using the nanomaterials based and nanotech-enabled products. For the benefits of human health and the environment, it is necessary to conduct a risk assessment on the production and the application of nanomaterials and nanotech-enabled products. Moreover, a uniform regulation of registering the nanotech-enabled products can help supervise the nanotechnology industry. For both risk assessment and product register system, the definitions of nanomaterials are the primary requirement. Based on an overview of the existing various international working definitions of "nanomaterials", a comparison analysis is conducted with respect to size, internal structure, physical and chemical characteristics. So far, seeking an inclusive size range and using a hierarchical approach are two significant methods to define the nanomaterials. However, neither approaches actually address the associated uncertainties when the definitions of "nanomaterials" are requested for the use as "enforceable terms" in a specific regulatory setting. A more detailed regulatory definition should be introduced for supervision. The other factors included in the definition must apply to all nanomaterials.