For most companies, just the spelling. The two are often used interchangeably, however a Jam (or Jamb) Nut is usually a thin version of a regular hex nut, whereas a Lock Nut covers a wider range of nuts which incorporate a locking feature. (plastic insert, Aerotight, Clevelok, etc)
Thanks for the reply. I found out that a Jamnut is basically a low profile hexnut which could be used to lock a second nut in place thus acting like a surface/ plate. I have used these terms interchangeably myself and was curious if there were differences. You explained the lock nut well enough.
