Billboarding is a technique commonly used during the Nintendo 64 era. It refers to the act of drawing characters or objects, then placing them in a three-dimensional environment. It's clear that the objects aren't 3D though nonetheless they appear to fit perfectly with the stage. It was done to both save memory and be economical since it didn't cost as much money. This technique was used in various popular Nintendo 64 games such as Mario Kart 64 and Super Mario 64, more-so the latter than the former.
Super Mario 64 - There are various things in Super Mario 64 that were drawn. The spheres that tumble down the mountain on Bob-omb Battlefield are all hand drawn, and no matter which direction you look at it, its appearance remains the same. The Bob-ombs and Wigglers are also all hand drawn.
Mario Kart 64 - In Mario Kart 64, all of the characters were drawn on a board. Various angles of each character were drawn. This was done because when eight polygonal characters were racing at the same the game had the potential to slow down. According to the developers of the game, the four-player battle mode of the game wouldn't be possible without billboarding.
"Necessity is the mother of invention. I love solving things like that. Because there wasn’t enough memory, thinking of an economical way to make the movements look right was like solving a puzzle, and I had a lot of fun." - Shigeru Miyamoto