shareholder y stockholder son lo mismo en inglés... depende de la preferencia de la empresa. El problema es que en castellano se diferencia entre acciones y participaciones, pero si el inglés es shareholder o stockholder puedes usar "accionista" con toda confianza.

Definitions of shareholder on the Web:

Any person who owns shares of a company's stock.
personal.fidelity.com/products/stocksbonds/content/ipoglossary.html

An owner of one or more shares of stock in a corporation
artrepreneur.thepauper.com/content.asp

An owner of a corporation whose ownership is represented by shares of stock in the corporation. A shareholder or stockholder has rights conferred by state law, by the corporation's by-laws or by a shareholders' agreement or buy-sell agreement. Some of these right include the right to be notified of annual shareholders' meetings, to elect directors and to receive an appropriate share of any dividends.
www.leanlegal.com/dictionary/s.asp

Definitions of stockholder on the Web:

Refers to the owner of a company who has contributed cash or some other form of consideration in exchange for an ownership interest.
www.theaccountspayablenetwork.com/html/modules.php

An individual who owns one or more shares of a corporation's stock, whether common or preferred stock. Stockholders may earn dividends and stockholders who have common stock have voting rights with regard to matters that affect the corporation.
www.tiaa-crefbrokerage.com/invest_glosry_StSz.htm

The holder of a number of stock certificates in a given company. See also Stock.
www.retirement-solutions.net/term.html