Tech tree
Also said as Skill tree, Research tree, Talent tree
A map of upgrades where each one requires those before it, so that buying one opens the next. You can't afford all of it, so your path through it is your strategy, and it is visible to you and often to your rivals.
It paces new abilities so that nobody gets the strongest thing first, and gives every player a long-term plan. Choices made early shut out others, which is what makes the plan yours.
Say it
“The tree is too wide to matter. Cut it to three branches, four nodes deep, with 6 points to spend by the end of the match, so nobody can finish two.”
Where you’ve seen it
Sid Meier's Civilization made the idea famous: you can't research Gunpowder without what comes before it, and choosing what to study next is one of the main decisions of a game.
Not to be confused with unlock list
A list opens things in a fixed order, and everyone gets them all. A tree branches, and what you take decides what you can't have yet.