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Ionosphere refers to the layer of the Earth's atmosphere that has a significant number of ionized atoms and electrons, due largely to photoionization by ionizing radiation (ultraviolet and X-rays) from the Sun. Sublayers of the ionosphere reside at altitudes also defined to be part of the thermosphere or exosphere, by the definition of these terms. Various sublayers of the ionosphere occur at altitudes that vary over time, due to atmospheric conditions as well as when and how much ionizing radiation Earth is receiving (it arrives during the day, and the resulting ions dwindle a bit through the course of the night). Also, when space weather from solar activity constitutes extra solar particles reaching Earth atmosphere, they also create (and contribute) ions: when these arrive at night, the resulting ionization can be visible to us, i.e., auroras. The ionosphere reflects some frequencies of radio, contributing to Earth atmosphere's pattern of blocked and non-blocked EMR frequencies (atmospheric windows). The reflections are used for communication, allowing reception of communication radio waves sent from beyond the horizon. The amount and type of reflection are affected by the ionosphere's variation.
The term ionosphere is also used for such layers of ions on other planets and moons, e.g., Titan.