incidence

/ˈɪnsɪdəns/
noun
  1. The rate or frequency at which something happens, especially something undesirable such as a disease or crime.
    • There has been a drop in the incidence of car accidents since the new traffic laws were introduced.
    • Researchers are studying the incidence of allergies in children.
    • The incidence of flu increases during the winter months.
  2. The way in which something falls or strikes something else, such as light or a projectile.
    • The incidence of the sunlight on the solar panels affects their efficiency.
    • The angle of incidence determines how much light is reflected.
    • Engineers calculated the incidence of the missile's trajectory.