Authorisation / Clearance Certificate
Plain English
Literally "no objection." It's a green light from an Italian authority that something is allowed to go ahead. The two you'll hear about most are the work nulla osta (an employer needs it before hiring you from abroad) and the family nulla osta (you need it to bring relatives over). The Sportello Unico in the Prefettura issues both.
Why it matters
An Italian consulate abroad won't release your work or family visa until the matching nulla osta has been approved in Italy. So it's the document that unlocks every step after it.
Real example
"When the Decreto Flussi quotas opened, Marco's restaurant in Verona applied online for a work nulla osta for a new chef. Six weeks later the approval landed at the Italian consulate in Dhaka, and only then could the chef apply for his visa."
Common confusion
It's not a visa and it's not a residence permit. It's only the pre-approval that lets the next steps happen, so don't book any flights based on it alone.
Official source
Ministero dell'Interno (Sportello Unico Immigrazione)