One speaks of "reflex" when one and the same stimulus always gives one and the same predictable answer.

A reflex can be innate or learned.
Examples of innate reflexes are the constriction of the pupil during lighting, pulling away a body part that is exposed to an unpleasant/painful stimulus and the so-called stretch reflex.
Examples of learned reflexes are that you start to blush and sweat when you lie.

Many reflexes can be blocked with an effort of will.