The crossword solution for the term 'SLEEK, INFORMALLY' is AERO, which is a four-letter word. AERO is an abbreviation of the word aerodynamic, which means having a shape that reduces air resistance and increases speed and efficiency. It is often used to describe objects that are smooth and streamlined, such as cars or airplanes. Therefore, AERO is an informal way of referring to something that is sleek.