A definition for global warming: An increase in global average surface temperature due to natural or anthropogenic climate change.

Global warming is closely linked to the enhanced greenhouse effect which is an increase in the concentration of greenhouse gases in the atmosphere leading to an increase in the amount of infrared or thermal radiation near the surface. Most scientists agree that the enhanced greenhouse effect is leading to rising temperatures, referred to as global warming, and other changes in the atmospheric environment, known as climate change (a term that in common usage also includes natural changes).

Encyclopedia of American History Gases created through human industrial and agricultural practices (primarily carbon dioxide from burning fossil fuels and wood, as well as methane, nitrous oxide, and chlorofluorocarbons) increase the heat-reflecting potential of the atmosphere, thereby raising the planet’s average temperature.

MSN Encarta Increase in world’s temperatures: an increase in the world’s temperatures, believed to be caused in part by the greenhouse effect.

Wikipedia Global warming is the observed increase in the average temperature of the Earth’s atmosphere and oceans in recent decades.