Complex sentence: Jackson's purchase also included all of Tennessee west of the Tennessee River, but in modern usage the term "Jackson Purchase" is used only to refer to the Kentucky portion of the acquisition; the Tennessee region directly to the south is typically called West Tennessee.
Split into simpler sentences: Jackson's purchase also included all of Tennessee west of the Tennessee River, but in modern usage the term "Jackson Purchase" is used only to refer to the Kentucky portion of the acquisition. The Tennessee region directly to the south is typically called West Tennessee.