The US Bureau of Labor and Statistics has just published a report with their final definition of a "Green Job":

Final BLS definition of green jobs.
BLS has developed this definition of green jobs for use in data collection in two planned surveys. Green jobs are either:
  1. Jobs in businesses that produce goods or provide services that benefit the environment or conserve natural resources.
  2. Jobs in which workers' duties involve making their establishment's production processes more environmentally friendly or use fewer natural resources.
For more details see: BLS definition of Green Jobs