Index fund
A mutual fund or ETF that tries to match a market index by holding the investments in it, rather than picking winners.
An index fund follows a set list, such as the 500 largest US companies or the whole US stock market. Because no one is picking investments, costs are often very low.
One fund can hold hundreds or thousands of companies, which gives you diversification in a single purchase. Its value still rises and falls with the market, and returns can be negative.