Expert
Expert is a Map difficulty, introduced in Flash version of Bloons TD 4, where it is the highest difficulty of maps. While it is not the highest difficulty for maps in the mobile version of Bloons TD 4 and all versions of Bloons TD 5, as Extreme are above Expert, it is the highest difficulty for maps in Bloons TD 6.
Mechanics
In BTD6, Heroes, Towers, and the player earn 30% more XP compared to in Beginner maps.