Breaking It Down
"Multi" means many.
"Channel" means a way into which something may be delivered or directed.
So, multichannel publishing means delivering a publication to your readers in many ways at the same time.
Multichannel Vs. Omnichannel
“...multichannel tends to be based on an inside-out approach, omnichannel is more outside in, a consideration of the customer experience from his perspective. ‘It's how a customer would see the world’”