The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. While the spatial size of the entire universe is unknown, it is possible to measure the size of the observable universe, which is currently estimated to be 93 billion light-years in diameter. In various multiverse hypotheses, a universe is one of many causally disconnected constituent parts of a larger multiverse, which itself comprises all of space and time and its contents as a consequence, (the universe) and (the multiverse) are synonymous in such theories.

Celtic music means two things mainly. First, it is the music of the people that identify themselves as Celts. Secondly, it refers to whatever qualities may be unique to the music of the Celtic nations. Many notable Celtic musicians such as Alan Stivell and Paddy Moloney claim that the different Celtic music genres have a lot in common. It is common for the melodic line to move up and down the primary chords in many Celtic songs. There are a number of possible reasons for this: Melodic variation can be easily introduced. Melodic variation is widely used in Celtic music, especially by the pipes and harp. It is easier to anticipate the direction that the melody will take, so that harmony either composed or improvised can be introduced: cliched cadences that are essential for impromptu harmony are also more easily formed. The relatively wider tonal intervals in some songs make it possible for stress accents within the poetic line to be more in keeping with the local Celtic accent.

War Thunder is based around combined arms battles on air, land, and sea. According to the game's creative director Kirill Yudintsev, it is the only game that encompasses all three of these experiences in one gameplay session. Vehicles range from the interwar period and the Spanish Civil War to the Iraq war and beyond, with an emphasis on World War II. Players can control aircraft, ground vehicles and warships from various nations, the major ones including the USA, Germany, Japan and the USSR. The game itself is divided into three main categories: Aviation, Ground and Fleet with game modes divided between Arcade, Realistic and Simulator that offer different styles of battle. In Aviation, gameplay is restricted to aircraft without helicopters whereas Ground Forces and Fleet will include their respective vehicle types of ground vehicles and warships alongside aircraft. A single player mode is available focusing on historical settings as well as a cooperative mode against AI ground vehicles and aircraft.