noun
votive tablet; ema
A small wooden plaque on which worshippers write prayers or wishes and hang at Shinto shrines or Buddhist temples. Originally depicted a horse as a substitute for donating a real horse.
神社で絵馬に願い事を書いた。
I wrote my wish on an ema at the shrine.
合格祈願の絵馬がたくさん掛けてある。
Many ema for passing exams are hung up.
From 絵 (e, 'picture') + 馬 (ma, 'horse'). Originally, real horses were donated to shrines, but over time painted wooden tablets of horses became substitutes, and later the tablets were used for various wishes.