A manuscript an editor didn’t specifically ask to see. Some publishers accept unsolicited manuscripts, but many do not anymore. Always check a publisher's writers' guidelines before sending an unsolicited manuscript. That is true whether you are submitting by mail or email. If you send an unsolicited manuscript to a publisher who has indicated he will not accept them, the manuscript will likely be ignored (if by email), returned without being read, or even discarded if sent by mail.
Even those publishers still open to unsolicited manuscripts are likely to simply add them to a slush pile that waits ignored until someone on the editorial staff has time to read them. Some book publishers have an entry-level employee read through the unsolicited manuscripts and pass any with promise along to the editor.
In most cases it's best to approach editors with a query letter or book proposal (check guidelines for preference) and if they ask to see your manuscript, be sure your cover letter and the outside of the envelope or subject line on your email indicates that it is a “Requested Manuscript.” For a list of book publishers who do accept unsolicited manuscripts, go to: http://joanyedwards.wordpress.com/2013/04/07/18-publishers-who-accept-unsolicited-manuscripts.