How your portfolio is divided among stocks, bonds, and other assets.
Asset allocation is the mix of asset classes in your portfolio and is the single biggest driver of long-term risk and return. A common guideline shifts from mostly stocks when young toward more bonds near retirement, so the portfolio is more stable when you need to draw on it. The right mix depends on your time horizon and risk tolerance.
An 80/20 allocation holds 80% stocks and 20% bonds.