Self-defense is a set of techniques and strategies used to protect oneself from physical harm. It can be used in a variety of situations, such as when facing an attacker or defending against an attack. The term 'self-defense' is often associated with martial arts, but it can also refer to any form of physical defense. The crossword solution for self-defense is 'actin, ' which refers to the actin proteins found in muscle cells. These proteins are responsible for muscle contraction and movement, and they are essential for self-defense.