mc_eid?Mailchimp's click-tracking tags: mc_cid names the campaign, and mc_eid is your personal subscriber ID, which identifies you, specifically, as the person who clicked.
This page covers both parameters: mc_cid (campaign ID) and mc_eid (your unique subscriber ID).
When a Mailchimp sender has click tracking on (the default), every link in the email gets two additions. mc_cid identifies the campaign, the same value for every recipient. mc_eid is different: it is your unique subscriber identifier, mapped in the sender's audience list to your email address, name, and whatever else they store about you.
That second one deserves emphasis. A URL carrying mc_eid is not just tagged as coming from a newsletter; it is tagged as coming from you. Paste it into a public forum and anyone operating the destination site, plus the sender, can connect the visit and the link to your subscriber record. Analytics tools on the destination can log it too.
Some sites use the token to prefill your email address into signup or preference forms, which is convenience built on the same identifier.
Yes for the page itself: it always loads without both parameters. Two small conveniences can disappear. A site that would have prefilled your email in a form will ask you to type it, and the sender's click statistics will not count you.
Given that mc_eid personally identifies you, stripping it before sharing a link anywhere is one of the clearest wins on this whole list.
Rather's Link Shortener strips mc_eid and hundreds of parameters like it, right in the address bar and in everything you copy or share. Free, open source, no account, no data collection, and the page you are on never breaks: the cleanup happens after the site has already loaded.