Data abstraction is the reduction of a particular body of data to a simplified representation of the whole.
Abstraction, in general, is the process of taking away or removing characteristics from something in order to reduce it to a set of essential characteristics. As in abstract art, the representation is likely to be one potential abstraction of a number of possibilities. A database abstraction layer, for example, is one of a number of such possibilities.
Data abstraction is usually the first step in database design. A complete database is much too complex a system to be developed without first creating a simplified framework. Data abstraction makes it possible for the developer to start from essential elements -- data abstractions -- and incrementally add data detail to create the final system.