Normal incidence refers to the angle at which a wave strikes a surface or boundary. It is the perpendicular angle of incidence that produces the least amount of reflection and the maximum amount of transmission. Some synonyms for normal incidence include perpendicular incidence, 90-degree incidence, right-angle incidence, or orthogonal incidence. These terms are often used in physics, optics, and acoustics when discussing the angle of a wave that is perpendicular to a surface. Normal incidence plays a crucial role in the behavior of waves and their interactions with their surroundings, making it an essential concept to understand in various scientific fields.