When the starting tribes come together halfway through the game
A Merge occurs when the 2-3 competing tribes come together to form one tribe. The merged tribe usually consists of 8-12 contestants. This also marks the halfway point in the season and is usually when the Jury phase begins.
Once merged, a contestant comes up with the new tribe name, color, and buff. From this point, the game changes from being a team-effort competition to a individual game. People then compete for individual Immunities and sometimes rewards.