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Pixel Art refers to the charming, simplistic, retro/renaissance style of very-low-resolution graphics, such as from the 1977-1995 Atari, GameBoy, & Super Nintendo (SNES) eras.

Science-fiction. Commonly including things like aliens, advanced technology, and settings of outer space or distant planets. 

Sprites are generally 2D images used in games to represent objects. They can be animated or static, and come in different styles like isometric or top-down.