The key difference between multi$-$channel and omnichannel retailing:

Omnichannel retailers concentrate on online and mobile channels only, while multi$-$channel retailers focus on traditional channels such as physical retail, catalog, pop$-$up kiosks etc.

Multi$-$channel retailers are cheaper because they focus only on a few channels.

Omnichannel retailers try to offer a more unified and coordinated shopping experience than multi$-$channel retailers

Multi$-$channel retailers have more than $3$ different channels; omnichannel retailers have fewer than $3$ channels