Although we marketers sometimes use "omnichannel" and "multichannel" interchangeably, the words refer to different marketing strategies and result in different customer experiences.
An infographic by marketing automation company Emarsys explains the difference:
• A multichannel marketing approach refers to a brand’s efforts to interact with customers across multiple platforms and channels. It focuses on growing customer engagement across those various channels.
• An omnichannel marketing approach, on the other hand, eclipses multichannel. Omnichannel marketing helps you promote stronger, lasting relationships with customers because they’re able to have a consistent brand experience no matter what device they use, when they use it, or where in their journey they are.
These two approaches work together when customers shop across multiple channels (multichannel marketing) and find that the experience is seamless (omnichannel marketing).
For more details on the differences between the two and how they complement each other, check out the infographic. Just click on the image to see a larger version.