Definition:
Many players think of blitz chess as a time control where each player receives 5 minutes (or less) for the entire game. The FIDE Handbook defines blitz as a game 'where all the moves must be made in a fixed time of less than 15 minutes for each player; or the allotted time + 60 times any increment is less than 15 minutes'.
Using a digital clock, if each player starts with 5 minutes on the clock and receives 5 additional seconds per move, the time per player would be calculated as
300 seconds + (60 * 5 seconds) = 600 seconds = 10 minutes.
This would be considered a blitz game. The FIDE Handbook defines a short game with more than 15 minutes per player as rapidplay chess.