Definition by the book

A shareholder, also referred to as a stockholder, is a person, company, or institution that owns at least one share of a company’s stock, which is known as equity. Because shareholders are essentially owners in a company, they reap the benefits of a business’ success. These rewards come in the form of increased stock valuations, or as financial profits distributed as dividends. Conversely, when a company loses money, the share price invariably drops, which can cause shareholders to lose money, or suffer declines in their portfolios’ values.

What it really means

A person or a company who owns portions (shares) in a company and therefore gets part of the company's profits and/or losses.