Can someone help me
 

what does this mean?

"sei". It means sex or gender. As in 男性 (male) or 女性 (female).

Thank you very much

性　is せい in hiragana and　is read sei sometimes, which means gender

By itself, it means sex, and I'm not talking about gender either.. err

HAHA! most ppl were right about the gender and sex part, but it also means characteristics， personality。

性格ー＞personality
自動性ー＞automatic nature, characteristically automated.

Of course it doesn't mean "sex" like "gender," kirakira. Don't you know that sex and gender are different concepts? Sex is a physical characteristic; gender is a mental construct. Gender - Wikipedia, the free encyclopedia

:)

Maybe in English. In other languages the word for sex and gender is the same...and by what kirakiraさん said it seems in Japanese too.

I'd say that differentiating between sex and gender is a stylistic/situational thing for english. If I want to know whether someone is a male or female, I'll use the words interchangeably. If I'm in a psychology class discussing 'gender roles', I would agree that gender is a mental construct. It can be tricky business trying to differentiate them because their differences are really only apparent (or relevant) when they're used in combination with other words. Certainly 'gender roles' and 'sexual characteristics' are two very different sets of concepts. However, if you're just talking about 'sex' or 'gender', then I think differentiating between them is a nit-picky, stylistic difference more than anything else.

No I was not talking about style or psycological aspect of it. I was talking about pure grammatical aspect of it. The word gender in some languages does not exist, it is used the word sex.
So in Italian to say have sex you use the word sex (fare sesso) and to say that a teacher is a male you say male sex (sesso maschile).
Same is for Japanese (I went check after Kirakiraさん mentioned it). To say a teacher is male you use the word 男性 and to say sexual relashionship you would use the phrase 性関係. Where in in English you would never say having gender with someone :)