The word up front in today's fashion vocabulary is ''lean.'' Often coupled with ''long,'' it describes the attenuated silhouette that has captured the attention of New York designers and their European counterparts as the linear direction worth following. Their top-rated styles for fall are narrow dresses, and slim jackets that end well below the hipline, paired with drawn-out skirts that reach to midcalf.
Nevertheless, the clothes are not always straight as a die. Such simon-pure examples of the lean and the long as Calvin Klein's exemplary suits do not ignore the natural curves of the body. Along with boxy shapes, there are many other semi-fitted ones with waistlines that are indented (or, as some say, ''suppressed''). Raised shoulders, slightly puffed sleeves, and flared peplums that emphasize the hips are not unknown, either. The perky jackets shown by Yves Saint Laurent in his couture collection were, in fact, cut very close in the midsection - which may well signify the coming of more fit in the future.
Women whose dimensions are antithetical to pencil-line clothes, or who regard wasp-waists and skinny skirts as forms of torture, should not despair. As has been the rule lately, one fashion trend does not make a season, and there are numerous alternatives to the elongated look.
Cropped jackets, bathrobe wraps, circular skirts, low-slung blouson tops, mini hemlines, loose chemise and smock styles, three-quarter-length toppers that are broad-shouldered and rectangular in shape, plus a variety of billowing ruffles and old-fashioned laces, are all parts of the fall picture.
Whether they are slender in outline or full and flounced, the majority of fashions have a retrospective feeling of one sort or another. Retail sales have been sluggish, and the economic climate is not favorable for innovative looks. The fashion industry has backed away from the experimental (the pant skirt, a modern concept that made practical sense and was the subject of some radical treatments, has virtually vanished). Instead, most designers have preferred to dip into yesterday's ideas and adapt them to today.
Clothes from the teens and twenties, so brilliantly presented last winter in the ''Brideshead Revisited'' series on TV and in ''Chariots of Fire'' in the movie theaters, are major influences in women's (and men's) fashions for autumn and winter. That era when styles were long and lean is the source for many low-waisted suits and dresses and for U-necked flapper modes, worn with drop earrings or a rope of pearls. Fair Isle and other traditionally patterned sweaters as well as the country tweeds and elegant velvets associated with upper-class British high-life have been reedited for 1982.
Input from other periods - Renaissance and Elizabethan, for example, and Victorian American - has been strong, too. Wide soft collars, pleated like a ruff, add a gently nostalgic touch to silk blouses. Handsomely done in jewel-toned prints to be worn with velvet or suede-trimmed spencers and swinging velvet skirts, such blouses are part of the Evan-Picone separates collection, aimed at the fashion-aware businesswoman.
Ralph Lauren, who has, with Calvin Klein, perhaps the most pervasive influence on what Americans wear of any US designer, continues to mine the realms of pioneer and Ivy League dress. His current clothes are more related to a Union Pacific parlor car than to a covered wagon. Few can afford to spend hundreds of dollars on a Lauren turnout of lace-trimmed linen blouse, antelope skirt, and handknit sweater (this year's have intarsias of petit-point roses or little red schoolhouses or trains). But versions of his prairie skirts, high-necked sheer cotton blouses with leg-of-mutton sleeves, worn with cummerbund sashes, are all over the middle-priced departments. He may also be credited with the upsurge in black-tie dressing.
The 1930s are echoed in the revival of the smoothly tailored, snug-waisted suit, a favorite with Perry Ellis as well as Calvin Klein. Those were the days, legend has it, when one perfect suit, a truly good handbag, some beautiful shoes , and a wardrobe of blouses, hats, scarves, and gloves were just about all an ultra-fashionable woman needed.
Suits - including jackets and skirts sold separately that can be paired off as suits - are having a banner year, both the matched and the unmatched types. Basing one's wardrobe on a couple of suits would be a sound approach this fall. The daytime selection goes from double-breasted chalk-stripe gray flannels to hearty tartans. (Scots plaids are insouciantly mixed together by Perry Ellis, who has cared more about color harmonies than about which particular clan is compatible with which.) Black is the outstanding choice for all hours, with bordeaux, teal, gray, olive, cinnamon, taupe, and violet well represented.
Dresses are also on hand in abundance, in one- and two-piece forms. Here again, black leads in solid-color choices, with gray flannel and autumn-leaf colored pincords runners-up. Low torso, chemise, and button-front shirt styles are some of the options.