A virtual database is a type of database management system that serves as a container to transparently view and query several other databases through a uniform API that culls from multiple sources as if they were a single entity. These databases are connected via a computer network and then accessed as if they are from a single database. A virtual database's goal is to be able to view and access data in a unified way without needing to copy and duplicate it in several databases or manually combine the results from many queries.
Virtual databases are also known as federated databases.