This is the 140th most frequent Italian word.
quel
"Quel" means "that" in English, used to indicate something specific.
The word 'quel' is used as a demonstrative adjective modifying 'libro' (book) and indicates a specific book being referred to in the context.
Quel libro sul tavolo è interessante.
That book on the table is interesting.
Here 'quel' is conjugated as 'quell’' due to the singular noun 'avventura' beginning with a vowel, still serving as a demonstrative adjective.
Non dimenticherò mai quell'avventura emozionante.
I will never forget that exciting adventure.
In this sentence, 'quel' functions as a demonstrative adjective associated with 'giorno' (day) to point out a particular day in memory.