What is a vested interest?

The term ‘vested interest’ is often used with a negative connotation, with regard to powerful and wealthy firms or groups who exploit their insider position or block policy changes that others believe would benefit the social interest, the latter potentially including future generations. But the term vested interests also covers members of the public who have rights to participate in public debate. So, how should we understand ‘vested interests’ for the purpose of improving and democratising policymaking processes?