What type of wave is commonly known as sky wave?

A sky wave refers to a radio wave that travels from the Earth's surface upward into the upper atmosphere, where it is refracted or bent back down to Earth by the ionosphere. This phenomenon is what makes long-distance, 'over-the-horizon' high-frequency (HF) communication possible. Because this refraction happens specifically in the ionized layers of the atmosphere known as the ionosphere, sky waves are accurately and commonly referred to as ionospheric waves. Other options are incorrect because tropospheric waves occur in the much lower weather-producing layer of the atmosphere (affecting VHF/UHF), and space waves travel strictly line-of-sight without returning to Earth.