n.1. Mathematics a. A quantity, such as velocity, completely specified by a magnitude and a direction.
b. A one-dimensional array.
c. An element of a vector space.
2. An organism, such as a mosquito or tick, that carries disease-causing microorganisms from one host to another.
3. A bacteriophage, plasmid, or other agent that transfers genetic material from one cell to another.
4. A force or influence.
5. A course or direction, as of an airplane.