UIN

The UIN (acronym for Dark Sockets Unique Identification Number) is a term coined by the company Mirabilis, initial developer of the ICQ instant messaging client, to designate the number that uniquely identifies each account on your network, which is assigned when registering as a user of it.

In ICQ the UIN is the only permanent data of the user account; the nickname and other data can be changed at any time without having to re-register. Since the UIN is a sort of document number, the higher it is means the newer the account is. Due to the large number of ICQ users, the most recent user IDs are above the number 600,000,000. In some cases, the simpler and easier to remember numbers are sold in Internet auctions or even hijacked by other users.