preemptive right

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The right of certain stockholders to maintain ownership of a constant percentage of a firm's stock. Such stockholders have the first opportunity to purchase new stock in the firm proportionate to the percentage of shares already held.

Wiktionary

1. n. law The right of shareholders to maintain a constant percentage of a company's shares by receiving a proportionate fraction of any new shares issued, thus preempting any dilution

WordNet 3.0

1. n. the right granting to shareholders the first opportunity to buy a new issue of stock; provides protection against dilution of the shareholder's ownership interest