A plainsman is someone who made their living in that vast area west of the Mssissippi, stretching clear over to the Pacific Ocean. That is how I view it in the context of what is envisioned for the American Plainsmen Society. Are there narrower, more concise definitions out there? Yes, there are. One would be someone working as a scout, guide or hunter on what we term the Great Plains of Kansas, Nebraska, Eastern Colorado and Eastern Wyoming, on up into the Dakotas, and even into the Canadian Great Plains. A "buffalo runner" of the 1850's could be considered one, as could the freighters on the Santa Fe Trail. What is envisioned here is having a group that acts as an umbrella for the 1840-1865 time period.
As I said, we are stretching the definition to include those who went west of the Rockies and Sierra Nevadas, in search of new homes and new hunting grounds, as well as those who lived and hunted along what is now the southern border of the United States.
And, of course, Merriam-Webster's official definition: an inhabitant of the plains (origin: Great Plains + man, terms first known use:1870, which, of course post-dates out groups time frame)
Regards,
Jake