The term ARM ART is composed of four words, each with a different number of letters. TAT (3 letters) is an abbreviation for the phrase 'tactical air navigation, ' which is a system used by the military to navigate aircraft. TATTOOS (7 letters) is the plural form of the word TATTOO, which is a form of body art that involves inserting ink into the skin. TATTOO (6 letters) is the singular form of the word TATTOO. ULNA (4 letters) is the name of the bone in the forearm, which is part of the arm. Together, these four words form the term ARM ART, which refers to the practice of decorating the arms with tattoos.