Herd Immunity can be defined as:
a) A cattle rancher's term for livestock granted protection from the slaughterhouse.
b) An inherited or acquired resistance to joining groups (e.g., glee club, sororities, the PTA, the Tea Party).
c) An exemption from bullying granted to middle-school students who do the bidding of high-status classmates.
d) A protection from legal liability granted to animal researchers.
e)When a critical portion of a community is immunized against a contagious disease, most members of the community are protected against that disease because there is little opportunity for an outbreak. Even those who are not eligible for certain vaccines—such as infants, pregnant women, or immunocompromised individuals—get some protection because the spread of contagious disease is contained. National Institute of Allergy and Infectious Disease
BTW, the "critical portion" of immunized people is typically given as 90 to 95% but depends on the disease.