What is the X in X-mas?
(CID) A short piece of text transmitted by some telephone systems describing the origin of a call, e.g. the name of the caller. Some telephone handsets can display this. A computer telephony integration system might use it to trigger actions on the callee's computer such as looking up the caller in a database and displaying their details on screen.
There may also be a separate "caller id number" giving the telephone number of the originator of the call.