What is nondiversifiable risk? How is it measured?

If not the returns of one-half the assets in a portfolio are perfectly negatively correlated along with the other half-which is very unlikely- some risk will remain after assets are combined into a portfolio. The degree of risk which remains is non-diversifiable risk, the part of a portfolio's total risk that cannot be eliminated by diversifying.

Nondiversifiable risk is calculated by a term known as beta (β). The ultimate group of diversified assets, the market, has a beta of 1.0.  The betas of individual assets and portfolios, relate their returns to those of the whole stock market.  Portfolios along with betas higher than 1.0 are comparatively more risky than the market.  Portfolios with betas less than 1.0 are comparatively less risky than the market.  (Risk-free portfolios have a beta of zero.)