It’s called an Ident-I-Eeze.
Here’s the relevant passage from the book, which comes in chapter 6:
[Ford] slowly drew out from the wallet a single and insanely exciting piece of plastic that was nestling amongst a bunch of receipts.
It wasn’t insanely exciting to look at. It was rather dull in fact. It was smaller and a little thicker than a credit card and semi-transparent. If you held it up to the light you could see a lot of holographically encoded information and images buried pseudo-inches deep beneath its surface.
It was an Ident-i-Eeze, and was a very naughty and silly thing for Harl to have lying around in his wallet, though it was perfectly understandable. There were so many different ways in which you were required to provide absolute proof of your identity these days that life could easily become extremely tiresome just from that factor alone, never mind the deeper existential problems of trying to function as a coherent consciousness in an epistemologically ambiguous physical universe. Just look at cash point machines, for instance. Queues of people standing around waiting to have their fingerprints read, their retinas scanned, bits of skin scraped from the nape of the neck and undergoing instant (or nearly instant - a good six or seven seconds in tedious reality) genetic analysis, then having to answer trick questions about members of their family they didn’t even remember they had, and about their recorded preferences for tablecloth colours. And that was just to get a bit of spare cash for the weekend. If you were trying to raise a loan for a jetcar, sign a missile treaty or pay an entire restaurant bill things could get really trying.
Hence the Ident-i-Eeze. This encoded every single piece of information about you, your body and your life into one all-purpose machine-readable card that you could then carry around in your wallet, and therefore represented technology’s greatest triumph to date over both itself and plain common sense.
Harl is the Guide executive, who introduces himself thusly:
“I am your new editor-in-chief. That is, if we decide to retain your services. My name is Vann Harl.”