LUNCH HOUR -- At the Eisenhower Theater through November 1.

In the glossary of the theater, the word for a pleasantly amusing play is "hilarious." This is in contrast to a play with lines that still seem funny when quoted afterward to people who did not see the show, which is called "brilliant," or one whose humor depends entirely on fast-paced stage business, which is called "zany."

"Lunch Hour," the new Jean Kerr comedy in a pre-Broadway engagement at the Kennedy Center, is neither brilliant nor zany, but fits the definition of being "hilarious." That is, it's good-natured good humor, just well-enough written and acted to be throughly enjoyable. But it doesn't quite make the sub-category of "something you can't afford to miss," which is the theatrical term for a show that certain people will be mentioning in party conversation for a season.

In the tradition of drawing-room comedy (which is the definition of plays in which people deal with adultery without getting angry), "Lunch Hour" stars Gilda Radner and Sam Waterson as the plain, awkward spouses of a pair of tall, suave, blond adulterers. The play concerns their attempt to "fight fire with fire" by faking an affair of their own.

Although the aggrieved husband is a marriage counselor by profession and all spouses make perfunctory complaints about not being "listened to," adultery is treated as a no-fault affliction that well-to-do householders have to expect, sort of like jammed garbage disposals or ants in the kitchen. The source of Kerr's humor, nicely sustained in Mike Nicholas' direction, is the general atmosphere of earnest, humble, self-blaming acceptance, in which it's acknowledged that diets and marriages are difficult to maintain.

Radner is remarkably good at this Just by whisking up the pieces of a plate she has broken, or worriedly eyeing a sack of dripping groceries in the midst of a dramatic confrontation, she puts an expert slapstick sealer on top of a richly funny mixture of pathos, anxiety and bumbling goodness. Waterson and Max Wright, in the role of everybody's friend, are also funny. But Radner is -- hilarious.