Thwack. The ball appears in a blur and ricochets off the side wall. You swat a low liner toward the corner. As the ball slams the hardwood wall, it spins suddenly and drops to the floor. Dead.

The point is yours. You take the game. Congratulations - now that's the way to play squash.

During moments like these, you can understand why squash has become an addiction for thousands of Washingtonians. Not just a fad, the sport inspires an all-consuming passion that leaves most players blind to inconvenience or poverty. Even with annual court rates as high as $600, the area's 15 squash clubs are busy from dawn to midnight.

The reason is apparent to any squash player. The game grows on you.

Squash is a sort of three-dimensional billiards match with overtones of tennis and racquetball. It involves bashing a hard-rubber ball around a boxlike white room with a racquet the size of a very large lollipop. But if you think that's easy, forget it. It's fast - a ball can zap off a wall at a hundred miles an hour - and requires the concentration of a chess game.A half-hour of squash is as demanding physically as two hours of tennis.

Yet squash is different from its racquet cousins in many ways. "It requires a longer racquet, smaller court and faster ball," says Geff Fisher, president of the National Capital Squash Racquets Association. "Squash is a harder game to learn, requiring more skills and hand-eye coordination. There's also more strategy involved in trying to psyche out the other player.

"Squash is to racquetball as chess is to checkers."

Fisher, 42, is a naval officer who became a squash fanatic seven years ago. Bored by jogging and angered by a growing paunch, he started working out at the Pentagon several times a week. Then in 1974 he helped found Washington's squash association to organize area-wide competition.

Nowadays the club boasts 300 members who play on the area's 35 courts. According to Fisher, most players are in the 20s and 30s, devotees of squash during the winter and tennis in the summer. The region's reigning champ is a 36-year-old psychiatrist. "The average player is probably a white-collar office worker," claims Fisher. "perhaps he's in government, but more likely in law or administration."

Still, squash wasn't always this accessible to the rank-and-file. Although orginally played in English prisons at the turn of the century, squash has often been called an exclusive "gentleman's game." In America, it flourished at New England prep schools and Ivy League colleges. Until very recently it remained there and at private social clubs - the domain of the Hasty Pudding set.

Today squash boasts a growing middle-class following. The National Squash Racquets Association estimates nearly half a million play weekly on 5,000 courts - an increase of 50 percent in the last five years. The 18' x 32' courts are turning up at local Ys and public rec centers throughout the District. In short, the sport has left snootier circles for the plain folks in cut-offs and sneakers.

So join the crowd.

There's nothing like pulverizing a ball after a tough day at the office. After deciding what pencil to sharpen all afternoon, it's a challenge to discover the infinite angle possibilities of a three-wall drop shot. Games are played to 15 points, with players scoring on unreturned hits. A match - the best of five games - requires dogged stamina to withstand the close, intense combat.

"It's a great little game," sums up Gene Rosen, a 25-year court veteran. "It's highly competitive and very rigorous." "You can get rid of the day's pent-up hostilities," adds Jeff Copeland, a Newsweek reporter who understands such things.

If there are any courtside complaints, it's that squash makes a lousy spectator sport. Even today, there are no big-money championships and few American clubs offer audience stands. In the District the University Club provides seating for most area championships. It seats 50. Otherwise, you have to stand atop an elevated platform and watch the action through wire mesh.

It could be worse. Ten years ago you couldn't even find courts to play on. By 1980 local builders promise 20 new courts for the metropolitan area. But for now, you may need a map to find the handful of public and commercial facilities. So don't vegetate waiting for tennis - find your way to a squash court and swing.