Some private equity firms invest... Private Equity Definition - Private equity (PE) is a financial buyer that invests in private companies of all sizes. Some private equity firms invest... Private Equity Definition - Private equity (PE) is a financial buyer that invests in private companies of all sizes. Some private equity firms invest across many industries, while others are focused on specific industries such as technology or energy services. They are a good alternative if you want to sell your company without inflicting severe and immediate change.