Ticks are small arachnids in the order Parasitiformes. Along with mites, they constitute the subclass Acarina. Ticks are ectoparasites (external parasites), living by hematophagy on the blood of mammals, birds, and sometimes reptiles and amphibians. Ticks are vectors of a number of diseases that affect both humans and other animals.

Despite their poor reputation among human communities, ticks may play an ecological role by ailing infirm animals and preventing overgrazing of plant resources. The American dog tick gets its name from the fact that adult ticks prefer domestic dogs as hosts, and this tick species is only found in North America.