Why is “Foster Child” not included?
There are other terms that mean “foster child”, such as 里子 (さとご), but 養子 doesn’t look like it does.
I didn’t know either, but it seems “adoptive child” and “foster child” are in fact not synonymous. “Adoptive child” is a child who has been adopted, while “foster child” is in general any child who isn’t being raised by their biological parent(s) (and consequently by someone else).
I realized I didn’t give enough detail to my answer, in light of a term that just came up in my reviews.
@obskyr makes the point, in that an adopted child and a foster child are different things. However, you may be thinking that 養子 should be “foster child” because WaniKani has the primary meaning of 養う set as “to foster”. In this case, it’s 養う that’s slightly off, as it doesn’t strictly mean “to foster”.
養う does mean “to foster,” it’s just that it’s broader than just taking care of children.
I find it an interesting reflection of old Japanese cultural norms that 養子, “adopted child”, also means son-in-law, where as 義理の娘, “duty” or “obligation” daughter, means daughter-in-law.
I guess since son-in-laws were “adopted”, they can’t be foster children.