(Service Profile IDentifier) A number assigned to an ISDN line by the telephone company that indicates which services the ISDN device can access. The SPID is used by the central office switch, and depending on the type of switch, either one or two SPIDs or none at all are required per line. The SPID is usually the 10-digit ISDN telephone number with a couple of extra digits tacked on. For example, the SPID for the ISDN number (215) 489-2077 might become 21548920770101.