What is the definition of "Global Warming"?
It is a good question.
It is generally taken to mean the increase in the Global Mean Surface Temperature (GMST) since the mid-20th century.
If you want to see more there is a wikipedia page:
http://en.wikipedia.org/wiki/Global_warming
Other definitions take the time scale back to the early 1800s.
A note of warning. The term "Global Warming" is often used synonymously with the term "climate change", but the two terms have distinct meanings. Global warming is the combined result of anthropogenic (human-caused) emissions of greenhouse gases and changes in solar irradiance.
Climate change refers to any change in the state of the climate that can be identified by changes in the average and/or the variability of its properties (e.g., temperature, precipitation), and that persists for an extended period, typically decades or longer.