Increase in temperature caused when incoming solar radiation is passed but outgoing thermal radiation is blocked by the atmosphere (carbon dioxide is the major factor) .

The warming of an atmosphere by its absorbing and reemitting infrared radiation while allowing shortwave radiation to pass on through.

Certain gaseous components of the atmosphere, called greenhouse gases, transmit the visible portion of solar radiation but absorb specific spectral bands of thermal radiation emitted by the Earth. The theory is that terrain absorbs radiation, heats up, and emits longer wavelength thermal radiation that is prevented from escaping into space by the blanket of carbon dioxide and other greenhouse gases in the atmosphere. As a result, the climate warms. Because atmospheric and oceanic circulations play a central role in the climate of the Earth, improving our knowledge about their interaction becomes essential.
nbsp;NASA Earth Observatory

Greenhouse Effect - The phenomenon in which outgoing infrared radiation that would normally exit from a planet's atmosphere but instead, is trapped or reflected because of the presence of the atmosphere and its components (see below) is called the greenhouse effect. It has been calculated that this effect is necessary to maintain the earth's climate and surface temperature and, more importantly, the liquid state of water in the majority of the earth's biosphere; however, the best scientific estimates to date suggest that increasing amounts of greenhouse gases are resulting in higher temperatures worldwide. This could result in melting of icecaps that would raise the sea level and cause devastating floods in coastal areas, more extremes in rainfall and intensity, and the distribution of species in the biosphere.
[The Greenhouse Effect; Matthew Kreljic, ed.; 1992; HW Wilson Co.; New York.]
[Climatic Change; v20: p. iii-vii: 1992.] 
Atmospheric Chemistry Glossary