I've heard and called a person 1.5 generation who immigrated to a foreign country during childhood(5-15years) and became the citizen. But is this "One point five generation" proper for such people? 2(second) generation are the people who were born by immigrants, but is 1.5 generation a common expression?
ex)He is Korean 1.5 generation. He immigrated to America along with his parents when he was only 10 and now he is American citizen.
While I like the concept, and I understand the meaning being conveyed, '1.5 generation' is not at all common in American usage. 'First generation' is used to refer to both immigrants as well as to the children of immigrants. Since that usage is ambiguous, 'second generation' is also used to mean two groups: the children of immigrants or the grandchildren of immigrants.
Taking the words themselves, since 'to generate' something is to create it, it is most logical to speak of immigrants, their chidren as the first generation (in the new country), and their grandchildren as the second generation (in the new country). Again, while I understand that '1.5 generation' would indicate someone who immigrated close to birth or in early youth, this would not adhere to the logical sequence. A person is either an immigrant or the offspring of an immigrant.
'1.5 generation' can also cause the same kind of confusion as the statement that the average couple has 2.5 children. Many people ask how can a couple have half a child.
I would call both the parents and the children immigrants. Both of them came from another country, age is not the issue. I can't think of any specific term to call the grandchildren - I would just call them by their names. Americans tend to not have such terms as once you become a citizen you are an American.
I haven't heard the term used in the UK.
When Koreans, from our position, call the 1st immigrants from Korea, we call them Immigrant 1st generation, and immigrant 2nd generation for the offsprings, and 1.5 generation for the in-between. But don't care about them, they are just Korean style terms.
So when you see Chinese, Japanese or Korean immigrants(1st or 2nd), and classify them in detail, what do you call them? Korean 1st generation or 2nd generation? There must be some terms related to ethnic immigrants, not general terms for immigrants.
I think this may be a cultural issue more than a language one- we don't tend to classify things this much. As immigration is a sensitive topic in the UK, labelling people by generation, or semi-generation, is not something we do much. We do have second generation, but subdividing it seems OTT- maybe they do it in specialised studies, but I have never heard it in everday speech or in a number of years working in the educational system (universities and colleges).
If the term 'second-generation immigrant' is actually used, then this post is rather irrelevant, but the expression seems to me to be strange. Children born in the country in which their immigrant parents have settled are not any kind of immigrants.
Context is always important; labelling is rarely important.