The year of the sweater

Comprehensive as that definition from The Fashion Dictionary may sound, it merely skims the surface when applied to sweaterdom today. Since 1973, when Funk and Wagnalls published the most recent edition of Mary Brooks Picken's excellent reference book, a whole undreamed-of world of sweaters has evolved. The lowly pullovers and cardigans of yesteryear have taken on the attributes of high-fashion attire, and the sweater's uses - as well as its dimensions - have expanded immeasurably.

Today, so many interpretations are represented that this is being called the ''Year of the Sweater.''

''A sweater,'' as designer Sal Cesarani has pointed out, ''can now be anything. A coat, a vest, a jacket, a dress.'' It may be added that a sweater can also now be worn anywhere at any time by anyone, so various are the colorations, shapes, textures, and materials.

The emphasis this season is on style. Utilitarian qualities have not been ignored, but certain important differences take this year's sweaters out of the purely functional class and put them on a lofty fashion plane.

To begin with, there's the matter of shape. Lines are loose and roomy. Dolman sleeves, deeply draped necklines, and bloused overlaps are favored. There's no sign of the tightly fitted body-encasing look reminiscent of the heyday of Lana Turner and the other legendary sweater girls.

While the classic forms are all on hand, they have undergone distinct changes. The new twin set consists of a very long cardigan over a short pullover. Fair-isle patterns have grown from the traditional area around the yoke to cover entire vests and jackets, and these are more amply proportioned. Turtlenecks and cowls come in more generous shapes, too. So do sweatshirt styles.

Evening sweaters - associated in many minds with those elaborate beaded, embroidered numbers people used to buy in Hong Kong - are a story in themselves. They may be quite simple, but when they are ablaze with glitter, sweaters for evening tend to strike a sportive note. Both Oscar de la Renta and Ralph Lauren use oversized argyle patterns in beading, or sequins as motifs on after-dark pullovers.

Other festive sweaters range from those with sprinklings of rhinestones over otherwise plain black to some rather sensational luxuries. Most of these evening sweaters are paired with a knee-length dinner skirt or pants, but they also work well with narrow long silk skirts.

Halston's twin sets for daytime feature large intarsia flowers woven into one shoulder. Pictorials and sweaters imitating the markings of rare animals (the most striking is the Perry Ellis zebra pullover) provide wit along with warmth. Ralph Lauren's ski-scene sweaters, which come in men's, women's, and children's sizes, will probably be as popular on city streets as on wintry slopes.

That a sweater can be anything these days is aptly illustrated by Calvin Klein, whose jackets take the form of short duffles with toggle closings and of sleekly tailored blazers. Knit dresses with a hand-fashioned appearance could easily go to the office (provided the thermostat is set at 65 degrees).

Angora, chenille, cotton, silk, Shetland, and boucle yarns are all employed, but cashmere's the star.