Wikipedia provides this definition: "extending of connectivity service in a location that is different from the home location where the service was registered"
Laymen understand it as using your cellphone on another carrier's network, instead of the one you pay for each month.
Beyond voice roaming that cell phones do, there is also 'data roaming' that happens with dedicated cellular data devices, like the EVDO modems from Sprint and Verizon.
All the major players have 'roaming agreements' that states the carrier whose customers had more roaming data consumed, ends up paying the other carrier a specified price per megabyte.
New rumors about Sprint data caps point to the possibility that Sprint is growing more and more concerned about the cost of data roaming:
Sprint reserves the right to limit throughput speeds or amount of data transferred and to deny, terminate, modify, or suspend service if usage exceeds 5gb per month in total or 300mb/month while off-network roamingFrom these rumors, its now evident just how quickly a customer can cost Sprint a lot of money... It wouldn't take long for an RVer to rack up over 300mb while roaming into a Verizon or Alltel network where Sprint does not have a presence.
Other news sites have made it clear that Verizon's recent purchase of Alltel Wireless made sense in many ways, one of which was how much money Verizon would save in roaming fees paid to Alltel. If that's the case, imagine how much worse it becomes for Sprint with one less partner to 'roam' with...