Ionosphere refers to a layer/layers of the Earth's atmosphere with a significant number of ionized atoms and electrons, the product of ultraviolet and X-rays from the Sun. The layers occur 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 differing altitudes that vary over time, i.e., due to atmospheric conditions and whether it is daytime, when the ionizing electromagnetic radiation arrives.
The ionosphere reflects some frequencies of radio, contributing to Earth atmosphere's the pattern of blocked and non-blocked EMR frequencies (atmospheric windows). The reflections are also used for communication, allowing reception of communication radio waves sent from beyond the horizon.
The term ionosphere is also used for such layers of ions on other planets and moons, e.g., Titan.