Step 1: Understanding the Term
The question asks for the definition of the Latin legal term "amicus curiae".
Step 2: Translating and Defining the Term
\textit{Amicus curiae} literally translates from Latin as "friend of the court".
In legal practice, an amicus curiae is someone who is not a party to a case (i.e., not the petitioner or respondent) but who assists a court by offering information, expertise, or insight that has a bearing on the issues in the case.
They typically file a brief (an "amicus brief") to provide a perspective that the parties themselves may not have presented. The court has the discretion to allow such submissions.
Step 3: Evaluating the Options
(A) Petitioner and (B) Respondent: These are the actual parties to the lawsuit. An amicus curiae is, by definition, not a party.
(C) Government Pleader: This is the lawyer representing the government in a case.
(D) A friend of the Court who assists the court: This is the literal translation and correct functional definition of an amicus curiae.
Step 4: Final Answer
An amicus curiae is a "friend of the court" who assists the court. Therefore, option (D) is the correct answer.