Power curve
Also said as Linear growth, Exponential growth, Progression curve, Scaling
How a player's strength rises over time, usually drawn against the same for enemies. Linear growth adds the same amount each step. Exponential growth multiplies by the same amount each step, so it starts slowly and then leaves everything behind.
The gap between the two lines is your difficulty, at every point in the game. Getting the shape wrong means enemies become either trivial or impossible, and no amount of tuning at level 40 can fix a curve that was wrong from level 1.
Say it
“Damage should grow 8% a level and enemy health 10%, so that fights get a little longer each level, and a player ten levels over can still lose to a group.”
Where you’ve seen it
Cookie Clicker raises each building's price by 15% with every one you buy, so prices double about every five. World of Warcraft had to shrink its numbers in Shadowlands, cutting the level cap from 120 to 60, after years of growth had made them absurd.
Not to be confused with difficulty curve
The power curve measures strength. The difficulty curve is how hard it feels, and that also depends on the player's skill, which grows without appearing on any chart.