A simultaneous submission is sending a single piece of writing--such as a poem, an article, an essay or a short story--to multiple publishers at one time for their consideration. The term is often confused with multiple submission, which is submitting multiple pieces to a single publication at one time.
Publishers that prohibit simultaneous submissions expect that writers will wait to hear from them before sending the piece to another publishing outlet. They want to be given an exclusive opportunity to review a piece. This is known as an exclusive submission.
Publications that accept simultaneous submissions request that writers notify them if their work is accepted for publication elsewhere so that those publications will not devote more resources to considering the work. Writers should keep careful records of what pieces they submit where, in case such notifications need to be sent.
In a difficult publishing climate, with submission volumes are high but acceptance rates low, submitting to multiple publications simultaneously may increase a writer's chance of being published sooner than submitting to one publication at a time.
Writers who rule out publishers that prohibit simultaneous submissions may limit their choices unnecessarily. Sometimes, outlets that accept only exclusive submissions respond more quickly, understanding that writers cannot proceed until they receive a response.