1) Computer memory unit 2) Unit of information 
The petabyte (symbol PB) is 1015 bytes of digital information. A related unit, the pebibyte (PiB), using a binary prefix, is 10245 = 250 = {gaps|1|125|899|906|842|624|bytes}, more than 12% larger. The prefix peta indicates the fifth power of 1000 and means 1015 in the International System of Units (SI), and therefore 1 petabyte is one quadrillion ...
One million megabytes, details ...
1,024 terabytes. 
petabyte (Pb) 1. A unit of computer memory or data storage capacity equal to 1 024 terabytes. 2. One quadrillion bytes. A typewritten page is said to have about 2 000 bytes of data per side; therefore, a megabyte represents a 500 text page per side and a petabyte would be 500 billion pages.
noun a unit of information equal to 1,024 terabytes