How is risk measured for a portfolio?

Question

How is risk measured for a portfolio?

Answer 1

Beta

Answer 2

return volatility relative to some market index benchmark

or

probability of losing money over a time period

or

the standard deviation of the portfolio's return

or

the likelihood of the deviation of an actual return from the anticipated return

There is more than one type of investment risk and more than one definition of risk, even for a portfolio of stocks.  However, all of the above definitions speak to the variability of the return of the portfolio, which has to be measured relative to a benchmark, and over a time period.  Risk is generally something to be avoided, diversified away, or tolerated in exchange for greater return.