incidence

Concise Oxford English Dictionary © 2008 Oxford University Press:
incidence/ˈɪnsɪdəns/

▶noun

• 1 the occurrence, rate, or frequency of a disease, crime, or other undesirable thing.

• 2 Physics the intersection of a line, or something moving in a straight line, such as a beam of light, with a surface.

• 3 the way in which the burden of a tax falls upon the population.

– origin ME: from OFr., or from med. L. incidentia, from L. incidere (see incident).