If you reinvest, you take money that has already grown from an investment — the extra money shares pay you, or the profit from selling something you owned — and you use it to buy more investments, instead of taking it out to spend. A person might reinvest what the shares paid, and so own a little more. A company might reinvest its profit in new machines, more stock, or another project. Though actually, people stretch it. The tidy sense is: put investment gains back into investments. In company talk, reinvest often just means do not pay the profit out to owners; put it back into the business, even when the new spend is not another buy on the market. The word lives in money pages and company reports. At home people more often say they put the money back in.