In object-oriented programming

Object-oriented programming

Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...

, a filter object is an object

Object (computer science)

In computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure...

which receives a stream of data as its input and produces the stream of data as its output based on the data from input stream. So a filter object filters an input data transforming it into output data.

Often the input and output streams are streams of characters, but these also may be streams of arbitrary objects. These are generally used in wrappers since they conceal the existing implementation with the abstraction required at the developer side.