A maturity model is a set of characteristics, attributes, or indicators that represent progression in a particular domain. A maturity model allows an organization or industry to have its practices, processes, and methods evaluated against a clear set of requirements (such as activities or processes) that define specific maturity levels. At any given maturity level, an organization is expected to exhibit the capabilities of that level. A tool that helps assess the current effectiveness of an organization, and supports determining what capabilities they need in order to obtain the next level of maturity in order to continue progression up the levels of the model.
Source: CERT RMM v1.2