hair既可作可数名词，也可作不可数名。作为可数名词时，含义为一根根头发、一根根皮毛，其复数形式为hairs；作为不可数名词时，指“头发”的总称，是集合名词。

1、作为名词时，意为汗毛，毛发，（尤指）头发;（动物的）毛，茸毛;毛状物，发毛织物;（植物叶茎上的）茸毛;些微，一点儿;

例句：I'm having my hair cut this afternoon.

我今天下午要去理发。

The rug was covered with cat hairs.

地毯上净是猫毛。

I wash my hair every night...

我每晚都洗头。

The majority of men have hair on their chest...

大多数男子长有胸毛。

I am allergic to cat hair.

我对猫毛过敏。

The stinging nettle has a square stem and little hairs.

荨麻长有四方茎和细小茸毛。

2、作为形容词时，意为毛发的，用毛发制的，保护毛发的。

例句：A fox terrier with smooth hair.

有光滑毛发的猎狐犬。

A terrier with wiry hair.

有金属丝般毛发的猎犬。

A small monkey ( Presbytis entellus) of southern Asia, having bristly hairs on the crown and the sides of the face.

长尾猴一种南亚产的头颈部和脸两边有直立毛发的小猴子（神猴叶猴属）

3、作为动词时，意为产生（或生长）毛发;形成毛状纤维;去除…的毛发。

例句：This was the kind of smile that made your hair stand on end.

这是那种让人不寒而栗的微笑。

She had a lot of make-up on and not a hair out of place.

她化了浓妆，精致得让人挑不出一丝瑕疵。

No one seems to turn a hair at the thought of the porced Princess marrying.

似乎没有人对离异的公主再婚感到丝毫地吃惊。

Don't split hairs. You know what I'm getting at.

别钻牛角尖了。你知道我指什么。