What it means
The public is the broadest possible term for the people who live in the society outside of your organization. The word is most often used in a political or geographical sense, to identify those who live in a particular state or a city, or who make up the communities in which your organization works.
How it’s used
The term public is often conflated with audience. They are closely related; however, audience is a more structured group of the public who is paying attention to your organization, targeted to receive a message, or receiving your messages and acting on them. In general, the public is the large and diverse group of all the people who could potentially be impacted by or intersect with your organization but may not yet be formally involved as members, donors, attendees, visitors, etc. It’s everyone we have the opportunity or potential to reach.
Many organizations also use the term in its plural form, publics, to identify multiple different broad groups that they are aware of. For example, if I am a national museum, my publics may be both local and spanning the country. These publics may not yet be audiences.
Why it matters
Almost all museums include as their mission a responsibility to the public, whether they are key audiences or not. The idea of public opinion and public perception, or the field of public relations, for example, are also built on the idea that what the public thinks, does, and says matters deeply to the success of an organization. Museums, on their most fundamental level, promote the public good.