What is mc_cid?

Who adds it

Mailchimp adds it to links in campaign emails when e-commerce tracking is on. It sits next to mc_eid.

What it means

The visit came from a link in a Mailchimp email. The value is the Mailchimp ID of the campaign that sent it, and stores can use it to credit orders to that campaign.

What to do with it

It names the campaign, not the reader. Leave it in place if you sell through a Mailchimp-connected store.

How Vireo shows it

Vireo doesn't store click IDs, in the free plugin or in Pro.

In the free plugin, Vireo drops the query string from page addresses, keeping only a few WordPress ones such as p and page_id, so a tagged URL counts as the same page as the untagged one. The visit is counted under the referrer it arrived with.

See Referrers and sources in the docs.

Sources

The mc_cid parameter is the Mailchimp ID for the campaign that generated the link.

E-Commerce - Mailchimp Developer

any links in that campaign’s emails will contain additional tracking parameters that can be used with the e-commerce API.

E-Commerce - Mailchimp Developer