Excess, Deductible, Retention, Co-Insurance, and Franchise

An Excess is the amount payable by the insured and is usually expressed as the first amount falling due, up to a ceiling, in the event of a loss. An excess may or may not be applied. It may be expressed in either monetary or percentage terms. An excess is typically used to discourage moral hazards and to remove small claims, which are disproportionately expensive to handle. The equivalent term to 'excess' in marine insurance is 'deductible' or 'retention'.
A co-insurance, which is typically applied in non-proportional treaty reinsurance, is an excess expressed as a proportion of a claim, e.g. 5%, and applied to the entirety of a claim.
A franchise is a deductible below which nothing is payable and beyond which the entire amount of the sum insured is payable. It is typically used in reinsurance arbitrage arrangements.