1) A poisonous alkaloid C10H14N2 that is the chief active principle of tobacco and that is used as an insecticide.

2) An addictive, poisonous chemical found in tobacco. It can also be made in the laboratory. When it enters the body, nicotine causes an increased heart rate and use of oxygen by the heart, and a sense of well-being and relaxation. It is also used as an insecticide.