Is composed of two or more separate elements; a mixture of principal and earnings. Investments that plow back or reinvest income grow at an accelerated rate. Earnings are generated on both capital and the accumulated income. 
Compound interest arises when interest is added to the principal, so that from that moment on, the interest that has been added also itself earns interest. This plowing back of interest into the principal amount, compounds the returns. 
Because of plowing back the income, returns compound on the sum of all past earnings as well as on the principal. For example, suppose $1000.00 (the principal) is used to buy shares in a closed-end company yielding 10 percent compounded annually. At the end of year 1, $100.00 is earned. At the end of year 2, the income is $110.00, $100.00 on the original principal and $10.00 on the interest - an so on in future years. 
As investors use the term, plowing back your earnings is expressed as total return. The change in value of an investment over a given period, assuming reinvestment of any dividends and capital gain distributions, expressed as a percentage of the initial investment.  The total return includes dividends, capital gains distributions, and the increase in the net asset value of the property. 
From 14c to put together, to mix, combine.