Ice is the solid state of water. The term can also be used to indicate an ice-like substance that is the solid state of some other material that is normally liquid or gas (e.g., dry ice) and in planetary science, the term is commonly used for frozen volatiles, typically those with a freezing point on the order of 100 K or higher (true of water, methane, and ammonia, but not nitrogen, hydrogen or helium). The substance is often used as an adjective to avoid ambiguity, e.g., water ice, or methane ice.
Ice (water ice) consists of water molecules (two hydrogen atoms covalently bonded to an oxygen atom) with hydrogen bonds (electrostatic bond, a bond between oppositely-charged ions) to the oxygen atom of another molecule. Ice is essentially water molecules stuck to each other (the type of adsorption called physisorption). Ice can be crystalline (showing a crystal structure) or amorphous (more random structure), the latter occurring when the water is brought to a very low temperature very quickly.
Ices (water and other types) and their detection are of interest in cosmology and in study of cool clouds as some molecules are expected to exist in the form of ice on dust grains in such clouds.