Definitions for "Herd immunity"
The immunity of a group or community. The resistance of a group to invasion and spread of an infectious agent, based on the resistance to infection of a high proportion of individual members of the group. The resistance is a product of the number susceptible and the probability that those who are susceptible will come into contact with an infected person.
Specifically, the mechanism by which an infection may be eradicated from a population although some susceptibles still remain, because the remainder of the population is immune and thus transmission is reduced. More generally, the immunological status of a population of hosts and its effect on transmission rates.
immunity characteristics of a population. Depends both on immune function of individuals within the population and on the rate of transmission amongst individuals.