Sine die is an adverb borrowed from Latin which literally means without day. It is used in English to describe something as not having a future designation. Another synonym is indefinitely.
Sine die is almost exclusively used in conjunction with the verb adjourn. It is a designation used in government organizations. If a meeting adjourned sine die, it means that there is no definite plan to meet again or that it is close indefinitely. If sine die is used without adjourn, it is still used in the context of the closing of a governmental session or conference.