Pratique is the license given to a ship

Ship

Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

 to enter port

Port

A port is a location on a coast or shore containing one or more harbors where ships can dock and transfer people or cargo to or from land....

 on assurance from the captain to convince the authorities that he/she is free from contagious disease

Contagious disease

A contagious disease is a subset category of infectious diseases , which are easily transmitted by physical contact with the person suffering the disease, or by their secretions or objects touched by them....

. The clearance granted is commonly referred to as Free Pratique.

A ship can signal a request for "Pratique" by flying a solid yellow square-shaped flag. This yellow flag is the Q flag in the set of International maritime signal flags

International maritime signal flags

The system of international maritime signal flags is one system of flag signals representing individual letters of the alphabet in signals to or from ships...

.