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A stock is a proportional ownership interest in a company. Or, in simplest terms, a stock is a portion of the value of a company. When you buy stock in a company, you are actually purchasing a piece of that company.
For example, let's say that we added up all of Widget Inc.'s assets, or the things that Widget Inc. owns—computers, office buildings, etc.—and the value of everything came to $1,000.00. Then let's say for argument's sake that Widget Inc. issues 1,000 shares of stock for people to buy. If you buy one share of Widget Inc., you own 1/1,000 of everything that Widget Inc. owns.