With the market itself taken to be the unique optimal portfolio of risky assets (a conclusion on which Sharpe, Treynor, Linter, and Mossin concurred"), the Capital Asset Pricing Model's parameter, beta, had a simple interpretation: it was the sensitivity of returns on the asset to overall market fluctuations. That sensitivity was the risk that could not be diversified away, and the extent of that "market risk" or "systematic risk" determined the relative prices of risky capital assets such as stocks.

Donald MacKenzie, An Engine, Not a Camera, loc. 253