The free fall in oil prices this year has introduced a new number into the national consciousness: the price of West Texas Intermediate.

The news reports tracking oil's decline this year usually quote the price of West Texas Intermediate, or WTI, in the shorthand of oil traders, one of the "benchmark" crude oils. (The other is of widely traded North Sea oil.)

At the beginning of the year, a 42-gallon barrel of WTI carried a $26.30 price tag. Yesterday, the cash price was a little more than $16.

What happens to the price of this particular kind of crude oil, lying beneath the dry range land and sand dunes around Midland and Odessa, Tex., has suddenly become a vital question for everyone connected to the energy industry. The oil called West Texas Intermediate rested undisturbed for at least 180 million years in an oil-rich geological region called the Permian Basin. A drilling boom after World War II uncovered the great size of these oil reserves and established it as a prime American crude oil.

Now, it is also an omen for the fate of the major oil companies, the wildcatters and independents; for banks, and for Third World debtors. Its fall promises lower gasoline and heating-oil costs for consumers, bargain prices for industrial energy users and a faster growth for the economy overall.

There are three prices, however, not one, for West Texas Intermediate.

The most volatile prices for WTI are set each day by traders and speculators buying and selling "paper" barrels on the New York Mercantile Exchange, the principal commodity market for oil in this country.

On the most hectic trading days since Jan. 1, the price of WTI on the New York Merc has fallen by more than $1 a day, and the closing price yesterday was $16.16 for delivery in March, down from $25.56 on Jan. 2 for delivery in February.

The "spot," or cash, price for WTI has fallen almost as far and as fast, from $26.30 a barrel on Jan. 1 to about $15.46 on Tuesday, and back up to $16.18 yesterday.

These two prices, particularly the futures price, have dominated the headlines and television reports of the price collapse.

The "posted" or contract price that governs most of the West Texas crude that actually gets turned into gasoline and other products hasn't fallen nearly as far. It now averages $24.38, down from about $28 six weeks ago, according to the Texas State Comptroller's Office.

There are three different prices because of the three different kinds of transactions -- futures, spot and contract -- which meet the differing needs of oil refiners, traders and speculators.

Oil refiners buy crude oil through long-standing contracts with oil producers who own the wells, paying what is called a "posted price." Typically, the contract guarantees that a refiner will receive a certain amount of oil on a designated day. The price isn't guaranteed, however. It is set, or posted, by the refining company.

A smaller number of refiners and traders buy oil daily on the spot market. This is the oil that isn't flowing under contract and thus is available for sale "on the spot."

Spot prices are reported in newspaper financial pages each day, based on surveys by companies such as the Oil Buyers Guide. Jay Amberg, senior editor for Oil Buyers, said he contacts five or six trusted oil industry sources -- refiners, traders, producers -- to get a consensus price on what West Texas Intermediate is actually selling for each day.

Dale Craymer of the Texas Comptroller's Office estimates that between 5 and 15 percent of West Texas Intermediate crude is sold on the spot market each day.

The third way to "buy" West Texas Intermediate is through a futures contract on the New York Commodity Exchange.

A refiner, trader or speculator will "buy" a contract, guaranteeing the right to receive 1,000 barrels of West Texas Intermediate at a predetermined date up to three months in the future. On the other side of the trade is a seller, promising to deliver that amount of WTI on the same date.

However, unlike spot and contract transactions, trades on the futures market almost never take place, according to the New York Mercantile Exchange. A trader committed to deliver oil under a futures contract can buy a contract to offset or cancel that obligation any time before the delivery date in the contract.

The prices of these contracts change every day, based on traders' notions about what West Texas Intermediate will be trading for one, two or three months in the future. Like other gambling transactions, these judgments swing on rumors and hunches. Yesterday, the possibility that the Reagan administration will accept a tax on oil imports helped to push futures prices upward.

In the three years since futures trading of West Texas Intermediate began, it has rapidly emerged as the best guide to daily oil prices in the United States, according to Peter Beutel, a trader with Rudolf Wolff Futures Inc. So many "paper" barrels are traded daily that the industry regards it as the most accurate guide to market conditions.

"The futures dictate the price," agreed Tony Mercandetti of Oil Buyers.

"The cash (spot) market for the past month has been absolutely subdued," eclipsed by the wild action in the futures market, said Amberg. Producers with excess barrels of WTI -- who ordinarily sell them for spot delivery -- now are holding onto them, waiting to see where oil prices will come to rest, Amberg noted.

The posted, or contract, price has moved downward more slowly, again because of caution by refiners in the face of collapsing prices.

In time, the contract prices will catch up with the futures and spot prices. When that happens, if contract prices fall low enough, the pace of oil production will begin to slacken, noted Dr. William Fisher, director of the Bureau of Economic Geology at the University of Texas in Austin.

The thousands of producers with small, marginal wells will shut down first, when prices fall near or below their production costs. Then larger, high-cost wells will do likewise. And eventually, the supply-demand pendulum will begin to swing again, away from glut, toward scarcity and higher prices, Fisher said.

When that happens, two or three years from now, perhaps, the turn will be signaled first on the futures market.