Definition 1

The climate change problem is related to changes in the concentration of the greenhouse gases (water vapor, CO2, CH4, N2O, and CFCs), which trap infrared radiation from the Earth's surface and thus cause the greenhouse effect. This effect is a natural phenomenon, which helps maintain a stable temperature and climate on Earth. Human activities, such as fossil fuel combustion, deforestation, and some industrial processes have led to an increase in greenhouse gases concentration. Consequently, more infrared radiation has been captured in the atmosphere, which causes changes in the air temperature, precipitation patterns, sea-level rise, and melting of glaciers.

source : http://web.ceu.hu/envsci/soe/problems/climdef.htm

Definition 2

Climate change refers to the variation at a global or regional level over time. It describes the variability or average state of the atmosphere or average weather over time scales ranging from decades to millions of years. These variations may come from processes internal to the Earth, be driven by external forces (e.g. variations in sunlight intensity) or, most recently, be caused by human activities.

source : http://www.global-greenhouse-warming.com/climate-weather.html