In addition to what the previous two answerers have come uip with, with which I concur, here is how a piece of American shipping legislation refers to it. It's just a fuller version of the same thing.

declaration [of shipment] under open [cargo] policy,

declaration on open policy, open cover declaration

(Source : GDT)
1 - open cover declaration = déclaration d'aliment n. f.

Note :
Pour assurer l'expédition, on peut prendre une police au voyage, mais si les expéditions sont fréquentes, l'assuré à intérêt à souscrire une police d'abonnement. L'assuré devra avertir la compagnie à l'aide d'un formulaire appelé déclaration d'aliment. Cette déclaration couvre toutes les expéditions faites par un armateur ou un chargeur.

2 - declaration of interest
Terme(s) apparenté(s)
declaration on open policy

déclaration d'aliment n. f.
Terme(s) apparenté(s)
avis d'aliment n. m.

(Source : Internat. Dic of Insurance and Finance, Chtd Instit of Bankers)

3 - Open cover = form of declaration policy used in marine insurance. Shipments are declared as and when they are made.

4 - Declaration policy = type of insurance policy in which the insured desclares details of the subject matter and sum insured under a policy at regular intervals.

5 - http://frwebgate.access.gpo.gov/cgi-bin/get-cfr.cgi?TITLE=46...

[Code of Federal Regulations]
[Title 46, Volume 8, Parts 200 to 499]
[Revised as of October 1, 2000]
From the U.S. Government Printing Office via GPO Access
[CITE: 46CFR308.512]
[Page 215-216]
TITLE 46--SHIPPING
CHAPTER II--MARITIME ADMINISTRATION, DEPARTMENT OF TRANSPORTATION
PART 308--WAR RISK INSURANCE--Table of Contents
Subpart F--War Risk Cargo Insurance
Sec. 308.512 Declaration of shipments under Open Cargo Policy.
(a) Closing report. (1) The assured shall file with the underwriting
agent, not later than the twenty-fifth day of each month, a closing
report for all inward shipments and a closing report for all outward
shipments, and pay the premium and fees, for all shipments covered
during the preceding calendar month, as required by Clause 19.

6 - http://www.marshcargo.com/glossary.htm

Declaration : Form filled out by Assured and sent to the broker and insurance company when reporting individual shipments coming within the terms of an Open Cargo Policy; usually used for declaring import shipments where evidence of insurance is not required.