Omnichannel - A Patient or HCP centric campaign that serves the right content, at the right time, via the right medium - delivering a seamless and connected audience experience via a holistic approach. An omnichannel campaign usually follows through different engagement touchpoints of a customer journey for example Awareness, Engagement and Advocate.
Multichannel - A campaign where the content is chosen based on the medium, and often unconnected. For example: Sales/Customer-facing team content vs Web content vs Ad content.