Controlling Interest

What Does Controlling Interest Mean?
When one shareholder or a group acting in kind holds a high enough percentage of ownership in a company to enact changes at the highest level. By definition, this figure is 50% of the outstanding shares or voting shares, plus one. However, controlling interest can be achieved with less than 50% ownership of the stock if that person/group owns a significant proportion of the voting shares, because in many cases, not every share carries a vote in shareholder meetings.

Investopedia explains Controlling Interest
For the majority of large public companies (such as those that belong to the S&P 500),  a shareholder with much less than 50% of the outstanding shares can still cause a lot of shake-up at the company. Single shareholders with as little as 5-10% ownership can push for their own seats on the board, or enact changes at shareholder meetings by publicly lobbying for them.