The term 'Diamond Head' is a reference to a volcanic tuff cone located on the Hawaiian island of Oahu. It is a popular tourist destination and is also known as Lēʻahi. The crossword solutions for this term are UMP (3 letters), DEE (3 letters), and MEE (3 letters). UMP stands for Unorganized Military Personnel, DEE stands for Diamond Head Education Endowment, and MEE stands for Military Entrance Exam.