$Input is a global variable whose value is the name of the stream from which input to Mathematica is currently being sought.
During the execution of <<file, $Input is set to "file".
During interactive input, $Input is "".
See The Mathematica Book: Section 2.11.4 and Section 2.13.1.
See also: Get, Streams, DirectoryName, ToFileName, $BatchInput, $ParentLink.