From The Aquarium Wiki
UILT - Upper Incipient Lethal Temperature
The upper limit of an animals environmental temperature that can be tolerated before dying.
Kept at the wrong temperatures, fish will become stressed and can fall ill or die. If the temperature is too close to the UILT they suffer from hyperthermia.
In many cases the LILT and UILT change as your fish mature, which is why fry and young fish are much more sensitive to changes.
- See also LILT