It refers to the single unified tax created by amalgamating a large number of Central and State taxes presently applicable in India.
The latest constitution Amendment Bill of December 2014 made in this regard, proposes to insert a definition of GST in Article 366 of the constitution by inserting a sub-clause 12A.
As per that, GST means:
Any tax on supply of goods
except taxes on supply of the alcoholic liquor for human consumption.
Note: And here, services are defined to mean anything other than goods.