No IANA registration; when it is open it is nearly always just another web server.
880 has no entry in the IANA registry and Nmap lists it as "unknown", though it shows up open somewhat more than neighbouring unregistered ports. There is no protocol associated with the number — its use is entirely ad hoc, most often an alternate HTTP listener or an appliance's management page picked to sit near 80 and 8080. Fingerprint it rather than assuming.
Expose with care
Usually an HTTP service that was moved off 80 rather than secured; obscurity is not access control, so check its authentication.
$ curl -sv -o /dev/null http://TARGET:880/Seen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.