Stylisation
Also said as Stylised art, Abstraction
Deliberately leaving out and exaggerating parts of reality so that what remains says more. A stylised tree is not a worse tree; it is a choice about which few things make a tree.
Realism needs enormous budgets and dates quickly, because the bar keeps rising. A strong style has its own rules, so it is cheaper to make and tends to age well.
Say it
“Stylise the trees: one trunk shape and two or three clumps of leaves in two flat values. No individual leaves. A player should recognise it at 64 pixels tall.”
Where you’ve seen it
After a realistic-looking Zelda tech demo at Space World 2000, Nintendo gave The Wind Waker flat colours and huge cartoon eyes. Fans were divided at the time, and it has since been remastered with that look intact.
Not to be confused with low quality
Stylised art is simple on purpose, and every remaining shape is doing a job. Low-quality art is simple because time ran out, and it usually still copies reality, just badly.