definition 1: (of a disease or illness) predicted to end up with death
definition 2: extreme (generally used when we refer to something negative)
other forms: terminal (n.)*, terminally (adj)
-One of our colleagues had terminal cancer.
-Celebrities organized a party for terminally ill children.
*Terminal as a noun means something totally different. It means an area at a station used by arriving or leaving passengers from buses, planes or ships. (ASTI has many terminals, but buses to Kastamonu always leaves from Terminal 1.)