Wiktionary
n. The underlying component that a database management system uses to create, retrieve, update and delete data.
Wikipedia
A database engine (or storage engine) is the underlying software component that a database management system (DBMS) uses to create, read, update and delete (CRUD) data from a database. Most database management systems include their own application programming interface (API) that allows the user to interact with their underlying engine without going through the user interface of the DBMS.
The term "database engine" is frequently used interchangeably with " database server" or "database management system". A 'database instance' refers to the processes and memory structures of the running database engine.
Usage examples of "database engine".
A new approach, called a spatial database engine, creates intuitive objects from standard geospatial databases and uses commercial databases to add attributes to the objects.