What is Bill Of Health?

Legal Definition
A bill of health is a document from officials of a port of departure indicating to the officials of the port of arrival which indicates whether it is likely that the ship is carrying a contagious disease.

As defined in a consul's handbook from 1879:

A bill of health is a document issued by the consul or the public authorities of the port which a ship sails from, descriptive of the health of the port at the time of the vessel's clearance. A clean bill of health certifies that at the date of its issue no infectious disease was known to exist either in the port or its neighbourhood. A suspected or touched bill of health reports that rumours were in circulation that an infectious disease had appeared but that the rumour had not been confirmed by any known cases. A foul bill of health or the absence of a clean bill of health implies that the place the vessel cleared from was infected with a contagious disease. The two latter cases would render the vessel liable to quarantine.
-- Wikipedia
Legal Definition
A certificate the sanitation authorities give an airport or seaport. If its clean than there is no contagious disease and people and edible cargo can be placed there. This prevents quarantine. AKA free pratique.
Legal Definition
; commercial law. A certificate, properly authenticated, that a certain ship or vessel therein named, comes from a place where no contagious distempers prevail, and that none of the crew at the time of her departure were infected with any such distemper.

2. It is generally found on board of ships coming from the Levant, or from the coast of Barbary, where the plague so frequently prevails. 1 Marsh. on Ins. 408. The bill of health is necessary whenever a ship sails from a suspected port; or when it is required at the port of destination. Holt's R. 167; 1 Bell's Com. 553, 5th ed.

3. In Scotland the name of bill of health, has been given to an application.made by an imprisoned debtor for relief under the Act of Sederunt. When the want of health of the prisoner requires it, the prisoner is indulged, under proper regulations, with such a degree of liberty as may be necessary to restore him. 2 Bell's Com. 549, 5th ed.
-- Bouviers Law Dictionary
Legal Definition
An official certificate, given by the authorities of a port from which a vessel clears, to the master of the ship, showing the state of the port, as respects the public health, at the time of sailing, and exhibited to the authorities of the port which the vessel next makes, in token that she does not bring disease. If the bill alleges that no contagious or infectious disease existed, it is called a "clean" bill; if it admits that one was suspected or anticipated, or that one actually prevailed, it is called a "touched" or a "foul" bill. In revenue law and procedure, the term is given to various documento filed in or issuing from a custom house, principally of the sorts described below.
-- Black's Law Dictionary
Legal Definition
A statement of a proper official certifying the healthful condition of a ship, its cargo and crew.
-- Ballentine's Law Dictionary