an eiderdown
eiderdown 作可数名词时指鸭绒被，复数形式为 eiderdowns。
He pulled an eiderdown over his knees.
他把一条鸭绒被拉到了膝盖上。
鸭绒被
鸭绒；绒鸭的绒羽,用于填充枕头和被褥；用这种绒羽填充的被褥
a soft quilt filled with the down of the eider duck
绒鸭的绒羽,用于填充枕头和被褥
The down of the eider duck, used for stuffing pillows and quilts.
用这种绒羽填充的被褥
A quilt stuffed with this down.
In winter, I sleep under a warm eiderdown.
冬天，我盖一条温暖的鸭绒被睡觉。
用法提示：eiderdown 是名词，指鸭绒被，这里作 under 的宾语。
She pulled the soft eiderdown up to her chin and closed her eyes.
她把柔软的鸭绒被拉到下巴处，然后闭上了眼睛。
用法提示：eiderdown 是名词，作 pulled 的直接宾语。
The old eiderdown had been in the family for years.
这条旧鸭绒被已经在家中使用了好多年。
用法提示：eiderdown 是名词，作句子主语。
eiderdown 作可数名词时指鸭绒被，复数形式为 eiderdowns。
He pulled an eiderdown over his knees.
他把一条鸭绒被拉到了膝盖上。
eiderdown 作不可数名词时指鸭绒材料，通常不加复数。
This cushion is filled with eiderdown.
这个靠垫里填充的是鸭绒。
duvet 是英式英语中最常用的说法，常带可拆洗被套；eiderdown 较老派且强调填鸭绒。现代日常对话中可互换。
quilt 含义更广，可指任何有夹层填充的被子，不一定是鸭绒；美式英语中多指绗缝床罩，替代时需注意上下文。
comforter 是美式英语，指厚而暖的盖被，通常不用被套，填料可为羽绒或化纤；与 eiderdown 语义相近但地域感不同。
grammar · medium
易错句子I bought eiderdown for the spare room.
正确表达I bought an eiderdown for the spare room.
当 eiderdown 指“鸭绒被”时是可数名词，前面需要冠词，所以用 an；只有指“鸭绒”这种材料时才作不可数名词。
grammar · medium
易错句子There are two eiderdown on the bed.
正确表达There are two eiderdowns on the bed.
当 eiderdown 指“鸭绒被”时，复数形式是 eiderdowns，因此 two 后面必须用复数。
eiderdown 特指用绒鸭绒填充的被子或绒鸭绒本身；duvet 泛指用羽绒或羽毛填充的厚被子，是更日常通用的词。
comforter 在美国英语中指普通厚被子，不强调填充物是否为羽绒；eiderdown 则强调鸭绒（尤指绒鸭绒）材质。
down 是羽绒这类材料的总称；eiderdown 既可指绒鸭的绒毛，也可指用这种绒毛填充的被子，含义更具体。
鸭绒被
She snuggled under the warm eiderdown quilt.
她钻进温暖的鸭绒被里。
鸭绒枕
She fluffed the eiderdown pillow before getting into bed.
她上床前拍了拍鸭绒枕。
柔软的鸭绒被
He sank into the soft eiderdown and closed his eyes.
他陷入柔软的鸭绒被里，闭上了眼睛。
在鸭绒被下
On cold mornings, I stay under the eiderdown as long as possible.
在寒冷的早晨，我会尽量赖在鸭绒被里。
用鸭绒填充的
The duvet is filled with eiderdown, so it is very light and warm.
这床羽绒被填充了鸭绒，所以又轻又暖。
复合词：eider（绒鸭）+ down（绒毛）；同根词：eider duck（绒鸭）、down jacket（羽绒服）
Dictionary facts from ECDICT, WIKTIONARY.