The money was lent for an undefined period of time.

这笔钱无限期借出。

A sort of undefined longing crept upon them.

一种莫名的渴望爬上了他们的心头。

The method XXX is undefined for the type yyy.

xxx方法对于类型yyy来说定义不明确。

Undefined reference, what was the solution last time?

未定义的引用，上次我们是怎么解决的呢？

The rest is in private hands, or its ownership is undefined.

其余的都是私人所有，或者所有权未定。