An editor who is not on anyone's payroll. Although most publications or publishers have in-house editors, smaller publications or publishers may have to hire freelance editors in order to keep up with the demand for editing skills. Freelance editors are called upon to perform various tasks, but with the advent of electronic submissions, those needs are changing. A good content editor must be very familiar with the house they are working for, and that house’s style, goals and specific needs. The content editor's duties would include determining what material needs to be included or deleted and when more might need to be added. If the freelance editor is hired as a copy editor, they deal with all the details, such as being sure the manuscript is ready to go to press; corrects the spelling, typos, and grammar; checks to be sure any facts, quotes, or references are correct, everything is in order and nothing has been lost or gotten out of order during the editing process.
There are also freelance editors who are available for hire by individual authors. In today's competitive book market, some authors go the extra step and hire a freelance editor to make their book as ready for publication as possible. In some cases, where there is competition between two or more books at the publishing house, it is the book that takes the least amount of work that gets selected. For tips on selecting a freelance editor, go to: http://janefriedman.com/2013/05/31/find-freelancebook-editor.