Yield is a term which describes the cash return on a security investment, and does not include appreciation.
Yield is the cash paid out of an investment in the form of dividends and interest received. The term does not encompass the appreciation of the investment, and it may be evaluated in different ways for different types of investments, so comparisons of yield across asset types is not standardized or recommended.
Many assets, from bonds to stocks to mutual funds, annuities, and more, have a cash yield. It is generally expressed as a percentage of the overall investment amount, by dividing the income received (even if its reinvested) by the total amount invested.
Reinvestment of dividends and so on, where possible, can add to the compounding effects of interest over time, and it is sometimes difficult to know whether the purported returns on an investment include calculations of dividends and interest payments reinvested.
Income strategies during retirement focus heavily on yield as opposed to returns and unrealized gains. The stocks of large, established companies often pay steady dividends with relatively low yields, while non-investment grade corporate bonds from mid- and small-cap companies tend to have a relatively high yield.
Municipal bonds and muni bond funds which avoid federal taxation on distributions might encourage investors to calculate their tax-equivalent yield, which is how much taxable yield they would have to get from another investment to equal the tax-free yield that their bonds pay out.
Some more examples of the various kinds of yield discussed in the financial world are current yield, yield-to-maturity, nominal yield, CD-equivalent yield, money market yield, effective yield, roll yield, repurchase yield, and so on.