noun, noun which may take the genitive case particle 'no'
ember; brand; stub (of used candle, match, etc.)
Refers to the remains of something that has been burning: a glowing ember, a charred piece of wood, or the stub of a candle or match after use.
キャンプファイヤーの燃えさしを水で消した。
We put out the embers of the campfire with water.
灰皿にマッチの燃えさしがたくさんあった。
There were many match stubs in the ashtray.
燃え殻 refers to ashes or cinders after something has completely burned, while 燃えさし implies a partially burned piece that still retains its shape or may still be glowing.
残り火 specifically means remaining fire or live embers, often with a poetic nuance, whereas 燃えさし is a more concrete term for the physical leftover piece.
Compound of 燃え (moe, the continuative form of 燃える 'to burn') + さし (sashi, from 差す 'to insert' or 'to remain', here indicating a leftover state). The spelling 燃えさし is the most common representation.