Beta
A measure of how much an asset moves with the broader market.
Beta describes sensitivity to the market. A beta of one moves with it, above one amplifies it, below one dampens it. It is the part of return explained by market exposure rather than skill.
Most of any portfolio's return is beta, market exposure, not skill. This reframes the cost question: paying high fees for what is mostly cheap, available market risk rarely makes sense. Knowing how much of a strategy is simply beta is the first step to understanding what you are really paying for.