Formally, backwardation means a downward sloping forward curve (as in an inverted yield curve). A backwardation starts when the difference between the future price and the cash price is less than the cost of carry.
The opposite market condition to backwardation is known as contango, in which the spot price is lower than the futures price. Different from contango, backwardation the difference between the cash price and the future price is unlimited. There have been situations in metal markets in the last 30 years where there was a contango in nearby quotes and a backwardation in two different futures quotes.
Backwardation very seldom, if ever, arises in money commodities like gold or silver, except one situation in the early 1980's when there was a one day backwardation in silver while some metal was physically moved from COMEX to CBOT warehouses .
The term is sometimes applied to forward prices other than those of futures contracts, when analogous price patterns arise. For example, if it costs more to lease silver for 30 days than for 60 days, it might be said that the silver lease rates are "in backwardation."
Some argue that backwardation is abnormal, and suggests supply insufficiencies in the corresponding (physical) spot market. This is empirically false: many commodities markets are frequently in backwardation, specially when the seasonal aspect is taken into consideration, e.g., perishable and/or soft commodities.
In Treatise on Money (1930, chapter 29), economist John Maynard Keynes argued that in commodity markets, backwardation is not an abnormal market situation, but rather arises naturally as "normal backwardation" from the fact that producers of commodities are more prone to hedge their price risk than consumers. The academic dispute on the subject continues to this day.
Backwardation is a normal market situation in a "sellers" market.
In the past on the London Stock Exchange, backwardation was a fee paid by a seller wishing to defer delivering stock they had sold. This fee was paid either to the buyer, or to a third party who lent stock to the seller.
The purpose was normally speculative, allowing short selling. Settlement days were on a fixed schedule (such as fortnightly) and a short seller did not have to deliver stock until the following settlement day, and on that day could "carry over" their position to the next by paying a backwardation fee. This practice was common before 1930, but came to be used less and less, particularly since options were reintroduced in 1958.
The fee here did not indicate a near-term shortage of stock the way backwardation means today, it was more like a "lease rate", the cost of borrowing a stock or commodity for a period of time.
In more recent years, a backwardation in equities quoted on the London Stock Exchange, has come to signify the unusual occurrence of an individual equities quote whereby the bid appears to be higher than the offer. This (of course) can not occur for electronically traded stocks via SETS or SETS MM but only for quote driven stocks (SEAQ)
Calendar spreads are known in LME jargon as "carries". Buying a carry is referred to as "borrow" and selling a carry as "lending". Thus, if a metal is subject to lending guidance, dominant position holders may be required to lend tom-next, that is sell a tom-position and buy a cash-position, should the backwardation for that period exceed a certain exchange-set percentage. The price differential and number of contracts to be lent is determined by LME regulation.
The LME uses backwardation limits in emergency situations, such as in 2005 following Hurricane Katrina when Zinc warrants in New Orleans were suspended until the warehouses were checked, or to act against possible or suspected market manipulation, such tightness in particular prompts for Aluminium in early 1999.