Billboarding
Also said as Billboard, Impostor, Camera-facing sprite
A flat picture in a 3D world that keeps turning to face the camera, so you never catch it edge-on. Trees, grass, particles and old-style enemies are often done this way.
One flat card costs two triangles, against thousands for a modelled tree, so a forest becomes affordable. The illusion breaks if you look straight down, or if you can see it swivel.
Say it
“Billboard the distant trees past 50 metres: one camera-facing card with two triangles, turning only around the vertical axis so they do not tip when you look up.”
Where you’ve seen it
Doom's monsters are flat pictures that always face you, drawn from eight angles so they appear to turn. Super Mario 64's trees are flat cards that turn to follow the camera.
Not to be confused with sprite
A sprite is the flat image. Billboarding is the rule that makes a flat image turn towards the camera in a 3D world.