Vector: In medicine, a vector is a carrier. The best way to understand a vector is to recall its origin as a word. Vector is the Latin word for a "bearer."

In parasitology (the study of parasitic organisms), the vector carries the parasitic agent. For example, in malaria a mosquito serves as the vector that carries and transfers the infectious agent (Plasmodium), injecting it with a bite.

In molecular biology, a vector may be a virus (or a plasmid); a piece of foreign DNA is inserted in the vector genome to be carried and introduced into a recipient (host) cell.

In physics, there are vectors but they go beyond the biomedical realm (except in cardiology).