What is wing aspect ratio and how does it affect aerodynamic performance?

Aspect ratio (AR) is the ratio of wingspan squared to wing area (AR = b^2/S) or simply wingspan divided by average chord. Higher aspect ratio wings have lower induced drag because they produce weaker wingtip vortices, making them more efficient for cruise (gliders have AR > 20, airliners 8-12). However, high AR wings are structurally heavier and less maneuverable. Fighter aircraft use low AR wings (3-5) for agility and high-speed performance.