Twenty-four of country music's most bankable stars lined up on the stage of Ford's Theatre last night, grinning hard as if posing for a high-school yearbook picture. With Dolly Parton placed strategically in the middle, in front of a smiling President Carter, this high-priced choir opened the "Celebration of Country" show with a song trying to explain what "Country Is . . ."

Barbara Mandrell explained that "Country is walking in the moonlight," while Johnny Cash added, somewhat paradoxically, that "Country is living in the city." With all the wildly diverse styles on stage, perhaps the most accurate line was delivered by Roy Clark: "Country music is what you make it."

What these singers have made of it is lots of money. Musical director Bill Walker admitted that almost all of them were chosen on the basis of scoring big hits during the past decade.

Musically, the evening had all the surprises of the "Greatest Country Hits" album sold on late night TV. Everyone did their big hit and exited. Or if they were more famous, they did a medley of their hits squeezed into five minutes. A few acts were tapped into the powerful currents that country music tradition contains. But many more played a nondescript form of pop music that strained any consistent definition of country music.

Commercial success was just about the only thing shared by a cast that included blue-eyed soul singer Barbara Mandrell, Tex-Mex singer Freddy Fender, former Beach Boy Glen Campbell and the gospel quartet, the Oak Ridge Boys. The level of talent ranged from genuine legends like Bill Monroe and Johnny Cash to musical imposters like Ray Stevens and Dottie West.

The evening's musical highlight was Bill Monroe & the Bluegrass Boys. As Monroe set up, Parton confided off-camera: "This is my favorite part." The 68-year-old Monroe, legitimately known as the Father of Bluegrass," sang "Blue Moon of Kentucky" in his haunting, mountain-chiseled voice.

Then Monroe led his quintet through an up-tempo string-picking version of the tune. He was the only act all night to be applauded by the 31-person orchestra of Nashville session musicians and local string and horn players.

There were other inspiring moments. Parton got more time than anyone, and she deserved it as her soprano shivered through her rural tales like "Coat of Many Colors." Freddy Fender closed his eyes and quivered melodramatically through Latin-tinged numbers like "Before the Next Teardrop Falls." Roy Clark proved his reputation as country's best guitarist with a rousing instrumental.

But much of the entertainment strained the definition of country music. Glen Campbell, Eddie Rabbitt, Mel Tillis, Lynn Anderson, Ronnie Milsap, Dottie West, Barbara Mandrell and Ray Stevens all sang a syrupy, bouncy music that was indistinguishable from any other kind of pop music. Most of the songs were also indistinguishable from each other.

Sen. Robert Byrd of West Virginia took the stage to play a fiddle tune with his backing quartet, Autumn Harvest. Byrd is a decent fiddler, every bit as good as the fiddlers that crowd the stage of any bluegras festival. But on "Our Little Cabin Home," he proved a woefully thin singer.

There was one major talent on the premises last night who never set foot on the stages. Mixing the sound for the TV show in a 40-foot tractor-trailer in the alley behind the theater was Scotty Moore. Moore was the 22-year-old guitarist who was instrumental in defining Elvis Presley's sound in 1954.

Moore identified the cause of last evening's failure to put forward a coherent definition of country music. "A lot of producers and record companies," he explained, "are kidding themselves or are kidding the people when they cut a pop record and call it country just because they stick a steel guitar in there. I don't think you can draw a line anymore between country and pop."