Pixel art
Also said as Sprite art, 8-bit style, 16-bit style
Art made at a low resolution with every pixel placed on purpose, then shown enlarged with hard edges. Each pixel is a decision, which is what separates it from a shrunken image.
It can be quick to make per asset and hard to make well. It only looks right if every asset shares one pixel size, so mixed pixel sizes are the classic mistake.
Say it
“Pixel art at 320 by 180, scaled up by whole numbers only. One pixel size for everything, with no rotated or smoothly scaled sprites, and characters 24 pixels tall.”
Where you’ve seen it
Small Mario in Super Mario Bros. is 16 by 16 pixels, and Shigeru Miyamoto has said he got his cap and moustache because hair and a mouth were too hard to show at that size.
Not to be confused with low resolution
Shrink a big drawing and you get low resolution: blurry edges and hundreds of accidental colours. Pixel art is built at that size, with a chosen colour in every square.