rise in temperature caused by gases or materials suspended in the atmosphere that trap energy from the sun.

Merriam-Webster Online Dictionary
greenhouse effect (noun)
warming of the surface and lower atmosphere of a planet (as Earth or Venus) that is caused by conversion of solar radiation into heat in a process involving selective transmission of short wave solar radiation by the atmosphere, its absorption by the planet's surface, and reradiation as infrared which is absorbed and partly reradiated back to the surface by atmospheric gases
greenhouse effect (Wikipedia)

The greenhouse effect occurs when greenhouse gases in a planet's atmosphere cause some of the heat radiated from the planet's surface to build up at the planet's surface. A planet is warmed by absorbing light from its host star and cooled by radiating energy into space. The warm surface of a planet emits longwave radiation. Greenhouse gases absorb some of that radiation, reducing the amount of energy that escapes into space. This reduction in planetary cooling raises the planet's average surface temperature. Adding greenhouse gases to the atmosphere increases the warming effect.

Greenhouse gases allow sunlight to pass through the atmosphere, heating the planet, but then absorb and re-radiate the longwave radiation (heat) the planet emits
Energy flows down from the sun and up from the Earth. Greenhouse gases intercept some of the longwave radiation emitted by the surface, preventing it from escaping into space, thus causing surface temperatures to rise.

The Earth's average surface temperature would be about −18 °C (−0.4 °F) without the greenhouse effect, compared to Earth's 20th century average of about 14 °C (57 °F), or a more recent average of about 15 °C (59 °F). In addition to naturally present greenhouse gases, burning of fossil fuels has increased amounts of carbon dioxide and methane in the atmosphere. As a result, global warming of about 1.2 °C (2.2 °F) has occurred since the industrial revolution, accelerating to a rate of 0.18 °C (0.32 °F) per decade more recently.

Greenhouse gases work by being transparent to wavelengths of radiation emitted by a star like the Sun, but absorbing wavelengths of radiation emitted by planets like the Earth. The wavelengths differ because matter radiates energy at a wavelength related to its temperature. The Sun is about 5,500 °C (9,900 °F), so it emits most of its energy as shortwave radiation in near infrared and visible wavelengths (as sunlight). The Earth's surface temperatures are much lower, so it emits longwave radiation (radiated heat).

The term greenhouse effect comes from an analogy to greenhouses. Both greenhouses and the greenhouse effect work by retaining heat from sunlight, but the way they do so differs. Heat is transferred into and through the air by radiation, conduction, and convection. Greenhouses mainly retain heat by blocking convection (the movement of air). The greenhouse effect retains heat by restricting the flow of longwave radiation through the air and into space.

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