Contretemps. C-O-N-T-R-E-T-E-M-P-S. Contretemps.

Definition: An inopportune, embarrassing occurrence or mishap.

Ironically, that was the word that made 12-year-old sixth-grader Molly Dieveney of Denver the champion yesterday at the 55th Scripps-Howard National Spelling Bee.

It was the heartbreak of psoriasis that finished off runner-up Uma Rao, 13, an eighth-grader from Pittsburgh. After Rao flubbed contretemps, a cool, deliberate Dieveney pounced on her chance, spelled it correctly and finished Rao off with an agonizingly enunciated psoriasis. Dieveney won $1,000, a gold trophy cup, a TV set and the Encyclopedia Britannica.

The word Dieveney chose to describe her Rocky Mountain high: E-C-S-T-A-T-I-C.

A record 126 elementary-school students, sponsored by their home-town newspapers, participated in the two-day event at the Capital Hilton. These were the survivors of more than 8 1/2 million kids who competed at the local level.

"I couldn't practice last week because I had walking pneumonia," said Dieveney. "I may not have looked nervous, but I really was." Tiny and preppy in a blue polo shirt and pink glasses, she became a favorite of the crowd when, after calmly asking the pronouncer 'Is that Japanese?' she unflinchingly spelled nisei, meaning an American or Canadian born of Japanese immigrant parents.

The champ, who announced plans to become an architect and child psychologist, was coached by neighbor Florence Bailly, whose son Jacques won the national title in 1980. "Molly was so excited when Jacques won that she camped out on his door till he came home, and it's been her goal now for two years," said Molly's mother, Linda Dieveney.

The 54 vocabulary whiz kids on the Presidential Ballroom's gold-and-blue dais looked like a typical seventh-grade class--braces glittering as they jittered and whispered under the glare of television lights. Most gulped down their fear and marched to the microphone as if to the guillotine, which was spelled correctly by Edward Sta. Maria of Jackson Heights, N.Y.

The four judges, educators all, wore headsets. Competition was halted several times to hear a taped "instant replay" of a controversial spelling. Misspellings were signaled by a tinny bell and the appearance of a sympathetic escort to help the vocabulary victim offstage with a hug and a kind word.

Words that will live in infamy for many this year: immiscible, avoirdupois, meunie re, Gruye re, luminophor, and almost any medical or scientific term. After Tom Finnerty Jr. of Sayre, Pa., grappled unsuccessfully with rappel, he sat dejectedly nursing a soda while other contestants buzzed around gathering autographs and exchanging addresses.

"I'm relieved that I don't have to stay up there, but I'm disappointed that I didn't do better," said Carol Lee Stott, 13, of Falls Church, who came in 63rd after missing bocaccio.

Secrets of the spelling stars:

* Long pauses before crucial vowels (one could see them forming on the lips). Trick vowels and silent diphthongs are killers in this war of the words.

* Word lists and books. "I was fortunate," said Jennifer Lynn Burns, 13, of Camden, N.J. "Someone sitting next to me yesterday had a national word list for the last 20 years and let me read it last night."

* Outright stalling. Julie Hilden, of Passaic, N.J., demanded the definition and etymology of marshmallow.

"Good training," said pronouncer Dr. Alex J. Cameron with a chuckle.