preemptive right

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The right of a company's existing stockholders to purchase enough shares to maintain their overall ownership percentage in the event of a new stock issue.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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