Interested Shareholder
A shareholder or association with beneficial ownership, whether direct or indirect, of enough voting stock to affect company decisions.
Investopedia Commentary
The actual percentage of stock that a beneficial shareholder must own is dependent upon the state or country that the company is headquartered in. The percentage may vary substantially.
Typically, the range is around 20% of all outstanding shares.
See also: Cumulative Voting, Outstanding Shares, Poison Pill, Shareholder, Takeover, Voting Shares