Titan is the only moon in the Solar System known to have a dense atmosphere. It is Saturn’s largest moon and has an atmosphere even denser than Earth’s, with surface pressure about 1.5 times that of Earth.
Titan’s atmosphere is composed mainly of nitrogen, with methane as the second-largest component and smaller amounts of hydrogen and complex hydrocarbons. Methane can evaporate, form clouds, fall as rain, and collect in rivers, lakes, and seas, creating a weather cycle that resembles Earth’s water cycle—though at a surface temperature near 94 kelvins, the liquid is methane and ethane rather than water.
Astronomers suspected an atmosphere because of limb darkening observed in 1903 by Josep Comas i Solà. The Cassini spacecraft later studied Titan extensively, while the Huygens probe became the first spacecraft to land on a moon in the outer Solar System in 2005.
The wording can cause confusion because several moons have extremely thin exospheres. Titan is the canonical answer because it is the only moon with a substantial, planet-like atmosphere capable of supporting clouds, weather, and a global haze layer.