That, my friends, is a simple, but profound question. It has no simple answer, and there is no 100% consensus on a definition, but let’s give a try at defining bluegrass music. Why do we need a definition? For the same reason we need definitions for all terms used in common discourse — so that we can communicate accurately. If I say “I love bluegrass music” and I mean Bill Monroe, the Stanley Brothers, and Dan Paisley, and you say “me too; it’s my favorite music” and you mean Trampled by Turtles, Railroad Earth, and Mumford and Sons, we haven’t really communicated, although we may think we have.
Historically, there is a consensus that “bluegrass music” began with Bill Monroe and his Bluegrass Boys band. The recordings made by this group in the mid to late 1940s are considered the gold standard of bluegrass music. What characterizes this music are:
1.   All the instruments are acoustic: an upright bass, mandolin, guitar, fiddle, and banjo.
2.   The singing is a “high lonesome” sound created through three-part harmony with a tenor or high baritone voice on top.
3.   There is great virtuosity on the instruments, and the instrumentalists take solos.
4.   The music is comprised of instrumental songs as well as ballads and story songs.
Before Bill Monroe there was Old Time Music, but it did not feature instrument solos or the three-part high-lonesome singing. There was blues from the Delta and Piedmont areas, but it did not utilize the instrument mix or singing strategy of Bill Monroe. There was fiddle or mountain music developed derivatively in Appalachia from the music imported from the British Isles, Italy, Africa, and elsewhere, but it, too, did not have many of the characteristics of Bill Monroe’s bluegrass music.
Bill Monroe listened to all of these styles of music, enjoyed them all, and his genius was to take aspects of all these forms and meld them into a new form: what we now call “bluegrass.” Pretty simple, no? If the music sounds like what Bill Monroe did, it’s “bluegrass” and we have our clear definition.
Music, however, is a living, breathing, ever-changing art form. Even Bill Monroe experimented with electric guitars, drums, and had a female bass player. Those who were contemporaries or early followers of Bill Monroe — the Stanley Brothers, Jim and Jesse, Flatt and Scruggs, and many others no doubt were influenced by the basic style of bluegrass created by Bill Monroe — each put their own spin on this new, evolving music. Although they changed what Bill Monroe had done in many subtle ways, most would agree their music is close to that gold standard and falls within the definition of “bluegrass.”
The next generation of bluegrass musicians pushed the envelope further — far enough that their music came to be called “Newgrass.” Newgrass Revival was a seminal group of this era (the 1970s) as was Old and in the Way. These groups used the same instruments Bill Monroe’s band had used in the 1940s (although John Cowan of Newgrass Revival played an electric bass) but they broke from tradition in many other ways, including the types of songs, the complexity of the chord structures, singing styles, and instrumentation. Newgrass became very popular and enjoyed considerable success.
From there the third generation of bluegrass musicians pushed the envelope even further. Blue Highway, Lonesome River Band, Mountain Heart, Alison Krauss, and many others put their marks on the music while still using the same basic acoustic instruments, some with an added Dobro. Groups like Alison Krauss and Union station took the music in a more pop-oriented direction with huge success. Hey, Alison Krauss has more Grammies than any female artist and the first million-selling bluegrass album.
If one compares what each generation produced with the music of their immediate predecessors one can see a gradual progression over time. But, if one compares the music of the third generation to the gold standard of the 1940s the change is dramatic. For some, too dramatic, and for these traditional purists this music is not “bluegrass.”
Now, we have the current generation of bluegrass musicians whose approach is wider and more varied still. In one sub-group we have bands like the Infamous Stringdusters, Yonder Mountain String Band, and others who still use the traditional instruments and still present recognizable aspects of earlier bluegrass music, but produce a more rock ‘n’ roll or jam band influenced version of the music. In a second sub-group we have bands like Trampled by Turtles, the Avett Brothers, Railroad Earth, and others who have pushed the envelope way further, still using banjos, mandolins, and fiddles, but electrifying their acoustic instruments sometimes adding drums and going pretty full-on rock ‘n’ roll to appeal to the younger generation.
With each progression of the music there are those who “fall off the wagon” trying to stand their ground that if it isn’t at least pretty close to what Bill Monroe did in the 1940s, it isn’t “bluegrass.” At the same time, there are large new audiences captured by the newer sounds. What all the performers of these versions of “bluegrass” have in common is a respect for Bill Monroe and the pioneers. The Avett Brothers love Bill Monroe. Yonder Mountain, too, as does David Grissman and the other modern practitioners. Bill Monroe was the innovator of his time, doing what no one had done before him, and these modern musicians, arguably, are doing the same — innovating and respecting that part of the Monroe tradition.
So, how do we communicate effectively in this morass of evolving perspectives and ever changing music? I suggest we refine our terminology. Let’s talk about “traditional bluegrass” and accept that means music in the style of by Bill Monroe, Flatt and Scruggs, and the other pioneers. Let’s talk of “newgrass” meaning the progressive music of the 1970s and ’80s that still has pretty close roots to the original. Let’s talk about “modern bluegrass” meaning music from the 1990s and 2000s typified by performers like Alison Krauss, Lonesome River Band, and Blue Highway, and let’s talk about “bluegrass derived music” meaning groups like Trampled by Turtles, Mumford and Sons, and the Avett Brothers, which really aren’t very close to what Bill Monroe did but still can trace their roots to the earlier forms of bluegrass music That’s not a perfect breakdown, but at least if you were to tell me you hate traditional bluegrass but love bluegrass derived music, I’d know what you were talking about!