To read a text and compare it with a source copy to detect errors.
In print publishing, proofreading is the final step before printing. It involves reading a proof (a preliminary version of the finished product) to ensure that it does not contain errors. Errors may have been introduced as part of the typesetting process, or they may be errors from the original manuscript which were not caught earlier.
In e-publishing, because an e-book can be viewed on a variety of devices, there is no such thing as "the" finished product. Instead, there are many possible reading experiences that will depend on the e-reading environment a reader uses.
For this reason, it is impossible to perform a direct proof-to-manuscript comparison as is done in traditional proofreading.
Instead, different aspects of proofreading will occur at different stages of the e-book production process. Some errors will be eliminated in the manuscript clean-up (standardization) phase, and more will be caught during validation and quality assurance.
Finally, field testing compares the text as viewed on a variety of devices against a theoretical ideal result.
Also see: copy editing.