Hi, Gadallah.

This term means, These circuits and system are friend to environment.

with my best regards

Green electronic devices and systems that consume less energy, thus lead to the reduction of global CO2 emission.

Hi maybe you can find the answer in this link:

http://www.ieee.org/conferences_events/conferences/conferencedetails/index.html?Conf_ID=16839

Exactly the circuits consuming low power and Eco-friendly are green circuits. In terms of Power systems power generation from Renewable energy sources in order to reduce the carbon emission. similarly electrical / smart / hybrid vehicles in  transport to reduce pollution. 

Usually, the 'green" term means an energy-efficient item.

for example: green circuits and green communication

http://www.apccas2014.org/cfp/APCCAS2014_CFP.pdf

http://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=5523682