Reflex

A reflex (/ˈriːfleks/) or reflex action is an automatic and fast movement in response to a stimulus. 
A true reflex is a behaviour done by the reflex arc. This is the path the signal of a reflex takes. That path is from the outside stimulus to the central nervous system (CNS), then the path from the CNS to the appropriate muscle. 
The nerve cells responsible for reflexes are not always in the brain, but often in the spinal cord. This way, reflexes are processed faster. Nerve cells in the brain still get feedback that the reflex action occurred. And, of course, reflex actions involving sight and sound are done by the brain. With invertebrate animals, the brain is not so dominant, and more is done by local reflex arcs in the segments.
Reflexes may take place even after death. The bite of a poisonous snake can be triggered up to several hours after it has died.
If the behaviour is complex, the stimulus is called a releaser, and the behaviour which follows may involve the whole animal for a long time in what is called a fixed action pattern. However, this is not the usual use of the term "reflex". It is studied in ethology because it usually involves two animals of the same species.