Reserved decision is a legal term. After the hearing of a trial

Trial (law)

In law, a trial is when parties to a dispute come together to present information in a tribunal, a formal setting with the authority to adjudicate claims or disputes. One form of tribunal is a court...

 or the argument of a motion

Motion (legal)

In law, a motion is a procedural device to bring a limited, contested issue before a court for decision. A motion may be thought of as a request to the judge to make a decision about the case. Motions may be made at any point in administrative, criminal or civil proceedings, although that right is...

 a judge may not immediately deliver a decision, but instead take time to review evidence and the law and deliver a decision at a later time, usually in a written form.

It is a more thought out decision compared to ex tempore

Ex tempore

Ex tempore has two meanings:-Performance:"Extempore" or "ex tempore" refers to a stage or theater performance that is carried out without preparation or forethought. Most often the term is used in the context of speech, singing and stage acting...

where a judge hands down a decision of a case soon or right after a hearing.