Green computing, green IT or ICT Sustainability, is the study and practice of environmentally sustainable computing or IT. In the article "Harnessing Green IT: Principles and Practices", San Murugesan notes that this can include "designing, manufacturing, using, and disposing of computers, servers, and associated subsystems—such as monitors, printers, storage devices, and networking and communications systems — efficiently and effectively with minimal or no impact on the environment."[1]
The goals of green computing are similar to green chemistry: reduce the use of hazardous materials, maximize energy efficiency during the product's lifetime, and promote the recyclability or biodegradability of defunct products and factory waste.
Many corporate IT departments have green computing initiatives to reduce the environmental impact of their IT operations.[2] Green ICT and its services present opportunities to deliver low carbon footprints and mitigate carbon emissions because of the unique ability to make energy consumption and green house gas emissions visible through its products and services.[3]
Definition - What does Green Computing mean?
Green computing is the environmentally responsible and eco-friendly use of computers and their resources. In broader terms, it is also defined as the study of designing, manufacturing/engineering, using and disposing of computing devices in a way that reduces their environmental impact.

Many IT manufacturers and vendors are continuously investing in designing energy efficient computing devices, reducing the use of dangerous materials and encouraging the recyclability of digital devices and paper. Green computing practices came into being in 1992, when the Environmental Protection Agency (EPA) launched the Energy Star program.

Green computing is also known as green information technology (green IT).
Techopedia explains Green Computing
Green computing, or green IT, aims to attain economic viability and improve the way computing devices are used. Green IT practices include the development of environmentally sustainable production practices, energy efficient computers and improved disposal and recycling procedures.

To promote green computing concepts at all possible levels, the following four complementary approaches are employed:
Green use: Minimizing the electricity consumption of computers and their peripheral devices and using them in an eco-friendly manner
Green disposal: Re-making an existing computer or appropriately disposing of, or recycling, unwanted electronic equipment
Green design: Designing energy-efficient computers, servers, printers, projectors and other digital devices
Green manufacturing: Minimizing waste during the manufacturing of computers and other subsystems to reduce the environmental impact of these activities
Government regulatory authorities also actively work to promote green computing concepts by introducing several voluntary programs and regulations for their enforcement.

Average computer users can employ the following general tactics to make their computing usage more green:

Use the hibernate or sleep mode when away from a computer for extended periods
Use flat-screen or LCD monitors, instead of conventional cathode ray tube (CRT) monitors
Buy energy efficient notebook computers, instead of desktop computers
Activate the power management features for controlling energy consumption
Make proper arrangements for safe electronic waste disposal
Turn off computers at the end of each day
Refill printer cartridges, rather than buying new ones
Instead of purchasing a new computer, try refurbishing an existing device