Submissions that are sent to more than one publisher at a time. (Also known as "multiple submissions.") Most publishing houses accept simultaneous submissions, but authors who choose this route should note this in their cover letters. A number of literary agents likewise accept simultaneous submissions. Submitting authors should always check applicable guidelines prior to mailing manuscripts.
Manuscript sent to more than one market at the same time.  Most markets do not accept simultaneous submissions; those that do will specify so in their guidelines.
The practice of submitting the same query or manuscript to many editors at once.