I could not find a published definition for Field Service Engineer -- besides, I would probably be better off describing the job in my own words:

Field Service Engineer [JOB]
A Field Service Engineer is a "Maytag" repair man for all types of equipment, ranging from simple laser printers to Internet Web servers. Most "Senior Field Service Engineers", have been involved or well exposed to strict troubleshooting patterns/paths. It takes a special type of technician to be a Field Service Engineer, because of the direct, heavy demands of the end-user, or equipment owner. We frequently experience the customer or end-user "venting" their anger or frustration over the situation. You must be able to juggle the priorities of a full day of service calls -- giving the highest priority to the customer who pays for "priority-one service."

The most difficult part of my job is not necessarily learning the skills to fix various hardware and software problems -- anyone can learn these with enough practice. The difficulty lies in diagnosing the problem when you are given very little information, resources, and time. Sometimes on a service call, when you're feeling the effects of Murphy's Law, it's like you're Joe Montana in the fourth quarter, on the fourth down, and if you don't throw a touchdown pass at that moment, your team is bound for defeat (i.e. you'll lose the customer account). Being a Field Service Engineer can be a very stressful job, but when you finally solve a problem, it's worth all the trouble (all the rude secretaries, all the long hold times on the phone with the manufacturer -- to get the answers right from the horse's mouth -- because nobody else wants to, and not to mention all the guys who claim to be able to do the job 100% better in 1/2 the time).

As I said, when you finally solve a problem, it's worth all the trouble.