M.Ted, Great example and great advice.

pdq, Trying to define terms here is an exercise often fraught with peril. There are always folks who don't want their belief systems disturbed and loudly insist that what they've always believed is in fact holy (wholly?) truth. Still, I'll give it a quick try.

• flatpicking: I'd say the term means the use of a flat plectrum to play intricate lead or melody lines on a six-string acoustic guitar normally in standard tuning. Pop and rock guitarists often use a flat plectrum or pick as well but their work is not normally described as flatpicking. I think an acoustic guitar is required to meet the definition.
  
  Sometimes jazz players using an acoustic guitar (e.g., Django Reinhardt) are described as flatpicking their instruments.
  
  I'm not sure I'd say that bluegrass rhythm necessarily fits the definition. Bluegrass rhythm is most often done using a flatpick today but it was originally done using a thumbpick and a fingerpick and could still be done that way today. Of course some great flatpickers also play rhythm in between solos and incorporate more complex work in their rhythm than could be accomplished with a thumbpick.
  
  
• bluegrass guitar: Bluegrass music is more of a genre than a technique. The techniques are constantly evolving and changing. A lot of well known flatpickers play old-time country tunes, jazz or even pop when they perform. Bluegrass is ensemble music with a particular rhythmic feel and blues emphasis. We had a long discussion some years back about what is bluegrass in a thread called The Genealogy of Bluegrass. About half way down, you'll find this definition 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.

  
  Others added to that definition in the thread but most folks agreed that it filled the bill.
  
  In bluegrass music, the guitar fills out the sound and provides rhythm. The rhythm isn't done using heavy chords but primarily with heavy moving bass notes. The rhythm guitar must be loud, and mellow with an emphasis on the bass. A high quality Dreadnought model is normally used. A bluegrass lead guitar is often less resonant but must also be loud. High quality Dreadnought models with Mahogany back and sides are often used here.
  
  
• crosspicking: That is what we are describing in this thread. It is a flatpicking technique in which the pick is used to play a pattern of eighth notes on (usually three) adjacent strings. But the strings need not be adjacent and as M.Ted has described, the patterns will be changed and moved throughout a tune.
  
  
• roll: This term actually comes from five-string banjo terminology. A crosspicking pattern is rhythmically similar to a three-finger "roll" on the banjo. So the various patterns employed in crosspicking are also referred to as rolls.

      - Mark