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What is meant by "Adult"?
In publishing, "adult" simply refers to books that are aimed at adults, as opposed to books that are aimed at children, or young adults, etc. It is not a euphemism for pornographic or erotic content. Nor does it necessarily refer to content which is unsuitable for children; it is just not targeted at them.
On this site, most ordinary mainstream publishers will be described as "adult", unless their books are specifically targeted at other groups (such as children, professionals, etc.).