Ultra High Temperature, according to my sources.
I have never understood the point of UHT milk. It might be milk in a technical sense, but unlike proper milk you can be pretty sure that anything you add it to will be actively unpleasant to eat or drink. As an alternative, soya milk lasts just as long, and while it isn't exactly delicious what little taste it has is inoffensive enough that cereal or tea made with it can actually be quite nice.
yclept says re UHT: Nonononono! It tastes carmelly and cooked. I used to chill it and eat it with a sweet cereal like a dessert. Sorta like eating vanilla pudding only thinner. Some things have a niche....
I can only assume that the esteemed yclept has been drinking some other kind of UHT, totally unlike anything you can get in Britain. Then again, BlackPawn insists:
re UHT: I've drunk British UHT milk and found it just fine. Admittedly it was already a low-fat version so I was accustomed to a different taste to regular full-fat milk anyway. I use UHT milk most of the time because finding fresh lactose-free milk is a pain and because I use milk irregularly and don't like it going off before I get around to using it.
I have to say I'm amazed, but there you have it. Maybe there have been some major innovations in UHT milk production in the last few years, or maybe this is just one of those no accounting for taste things.