Terrific is an adjective used to describe something or someone has extremely large or extremely good. It carries a good connotation.
Archaically it meant to incite fear or terror.
The adverb form is terrifically and is usually used as a submodifier.
Terrify is a verb that means to cause fear in others.
Terrifying has replaced terrific as the adjective that means to cause fear or terror. The adverb form is terrifyingly.
Terrified, on the other hand, is an adjective that means to be scared or fearful.
A person who terrifies is a terrifier.
“He is a terrific director and will always remain special to me,” she said of her first director. [The Indian Express]
There’s also a terrifically nasal imitation of Kenneth Williams and a particularly funny sketch recalling the first time he recorded Harry Potter. [The Telegraph]
A 2-year-old girl was seriously injured Wednesday when she took a terrifying plunge from the third-story window of her Brooklyn apartment and landed on a brick courtyard below, authorities said. [New York Daily News]
Should Alberta business be terrified of this new all-orange New Democrat government? [Edmonton Sun]
The terrified woman was sitting in the middle seat with the angry man in the window and she asked the passenger sitting in the aisle to help her flag down a flight attendant. [SF Gate]
Where the original — published under the Richard Bachman pseudonym in 1979 — was a page-turner and a terrifier, a sequel could give us a wider angle on the nation that has sanctioned a televised marathon that uses teenage boys as bullet depositories. [Grantland]