What is big data?

The best definition I've found in the net:

Big Data is a term used to describe large collections of data (also known as datasets) that may be unstructured, and grow so large and quickly that it is difficult to manage with regular database or statistics tools.

SAS - Equivalences between PROC SQL joins and DATA statements

It is explained here (in Spanish) :

http://analisisydecision.es/proc-sql-merge-set/