O nome distinto de partição de diretório em uma floresta do AD DS. Contextos de nomeação incluem três contextos de nomeação de leitura/gravação — configuração, esquema e domínio — e servidores de catálogo opcionais somente leitura contextos de nomeação que estão presentes nos controladores de domínio são globais. Um contexto de nomenclatura também pode ser uma partição de diretório de aplicativo. Especificar um contexto de nomeação como um nome distinto, que indica a relação hierárquica para o domínio raiz da floresta, por exemplo, DC = Meu_domínio, DC = Contoso, DC = Com.
https://technet.microsoft.com/pt-br/library/cc770963(v=ws.10).aspx
Due to the distributed nature of Active Directory, it is necessary to segregate data into partitions. If data partitions were not used, every domain controller would have to replicate all the data within a forest. Often it is advantageous to group data based on geographical or political requirements. Think of a domain as a big data partition, which is also referred to as a naming context (NC). Only domain controllers that are authoritative for a domain need to replicate all of the information within that domain. Information about other domains is not needed on those domain controllers. On the other hand, there is some Active Directory data that must be replicated to all domain controllers within a forest. There are three predefined naming contexts within Active Directory:
1) Domain naming context for each domain
2) The Configuration naming context for the forest
3) The Schema naming context for the forest
Each of these naming contexts represents a different type of Active Directory data. The Configuration NC holds data pertaining to the configuration of the forest (or of forest-wide applications), such as the objects representing naming contexts, LDAP policies, sites, subnets, Microsoft Exchange, and so forth.
The Schema NC contains the set of object class and attribute definitions for the types of data that can be stored in Active Directory. Each domain in a forest also has a Domain NC, which contains data specific to the domain—for example, users, groups, computers, etc.
Fonte: Active Directory 5ª Edição. Página 63.