Tiers of play are groupings of levels that identify the overall power and influence of an adventurer or party. In the current edition of the game, the tiers are divided as: Levels 1 - 4, levels 5 - 10, levels 11 - 16, and levels 17 - 20. Unlike 4th edition, these tiers do not have names, but are simply identified as "FirstTier," Second Tier," etc.
The text of the Player's Handbook identifies the First Tier as "apprentice" and implies the Second Tier could be understood as "Master" ("characters come into their own"). Building from these, Third Tier is "Grand Master," and Fourth Tier is "Paragon."
5th level as the beginning of Master tier supports my theory (introduced in this post) that "Name Level" since 3rd edition has been 5th level.