A mixture of traditional and self-publishing. This is an evolving category that lacks a strict definition. You can think of a hybrid publisher as one who asks you to shoulder much or all of the costs, but is very selective in what it publishes. Since they don't publish any old author, hybrid publishers may do a better job with distribution and marketing. Some hybrid publishers work on a crowdfunding model, insisting that you develop a following and get a certain number of pre-orders before they will publish your book.