A security representing a fractional ownership stake in a company.
Owning stock makes you a part-owner of a business. That usually comes with a claim on distributed profits (dividends), a vote at the shareholder meeting and a share of whatever is left if the company is wound up.
Shareholders’ liability is limited to their investment: the stock can become worthless, but you cannot lose more than you paid. Over the long run, stock returns come from earnings growth, dividends and changes in valuation.