A fully concealed hand in Mahjong Island is a winning hand with no Open Groups. Fully Concealed is a Mult Hand, and it adds +3 Mult to your win.
To win, you complete one hand on the Island Card, Mahjong Island's own card of hands. That means 14 tiles that match one line on the card. Fully Concealed rewards you for keeping that whole hand on your Rack.
How does a win earn Fully Concealed?
A win earns Fully Concealed when none of its Groups are open. A Group is made of identical tiles, like a Set (3 matching tiles).
Here is how it changes your score. Every win scores Chips × Mult, and Mult starts at 2. Fully Concealed adds +3, so Mult goes from 2 to 5.
It also stacks with other Mult Hands. Draw your own winning tile and Self-Drawn Win adds +2 more. Win with no Jokers and Jokerless doubles your Mult (×2). See the full order in how Mahjong Island scoring works.
One tip for building toward it
Aim to finish from the Draw Pile. A hidden hand you complete yourself earns both Fully Concealed and Self-Drawn Win. That is +3 Mult and +2 Mult together. The post on when to take a discard helps you weigh that choice.
Some Jokers reward a hidden hand too. This month's Teacup Joker scores ×2 with no Open Groups.
Want to see how Mult Hands add up in practice? The lesson Growing your Mult shows each step.