I found these definitions in 14 CFR Part 1.
Maintenance means inspection, overhaul, repair, preservation, and
the replacement of parts, but excludes preventive maintenance.
Major alteration means an alteration not listed in the aircraft,
aircraft engine, or propeller specifications--
(1) That might appreciably affect weight, balance, structural
strength, performance, powerplant operation, flight characteristics, or
other qualities affecting airworthiness; or
(2) That is not done according to accepted practices or cannot be
done by elementary operations.
Major repair means a repair:
(1) That, if improperly done, might appreciably affect weight,
balance, structural strength, performance, powerplant operation, flight
characteristics, or other qualities affecting airworthiness; or
(2) That is not done according to accepted practices or cannot be
done by elementary operations.
Minor alteration means an alteration other than a major alteration.
Minor repair means a repair other than a major repair.
http://a257.g.akamaitech.net/7/257/2422/14mar20010800/edocket.access.gpo.gov/cfr_2002/janqtr/14cfr1.1.htm
Should be more help than my first post.