Here's a definition by Mark Clark from a recent thread named "Genealogy of Bluegrass:"

The term bluegrass, referring to a particular style of American country music, is derived from the band name of its progenitor, Bill Monroe and his Blue Grass Boys. Proud of being a Kentuckian, Monroe named his band for "The Blue Grass State." Bluegrass music is acoustic ensemble country music characterized by driving rhythms, separate and distinct parts for each instrument and voice, improvised solos and strident high harmony singing with the tenor often placed a fifth above the lead or melody voice. The primary instruments found in bluegrass music are fiddle, mandolin, 5-string banjo and Dobro (resophonic guitar played with a slide) with guitar and double bass added for a rhythm section. Occasionally, a second guitar is added as a lead instrument. A uniquely bluegrass style of playing has evolved for each of the instruments that is generally distinct from the way that instrument is used in other musical styles. Created in 1945, bluegrass music draws heavily on older forms including blues, jazz and the earlier string band music of the Southern Appalachian Mountains. Today, bluegrass music is played in countries all around the world with a faithfulness to the original form that is ofen astonishing. Mark Clark

I'd still like more info and feedback. Does this define bluegrass?
What about the 5-string banjo played 3-finger style, is that the difference between old-time and bluegrass?

I don't think that "Appalachian Folk Music= Bluegrass." There are too mnay other influences. What about songs like"Bill Bailey," "Home Sweet Home" or "Alabama Jubilee," they certainly aren't Appalachian Folk Music.

-Richie