- 1a archaic : reflected heat, light, or color
- 2a : an automatic and often inborn response to a stimulus that involves a nerve impulse passing inward from a receptor to a nerve center and thence outward to an effector (as a muscle or gland) without reaching the level of consciousness — compare habit
- b : the process that culminates in a reflex and comprises reception, transmission, and reaction —called also reflex action
- c plural : the power of acting or responding with adequate speed
- d : a way of thinking or behaving
- 3: a linguistic element (as a word or sound) or system (as writing) that is derived from a prior and especially an older element or system <boat is the reflex of Old English bāt>
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. In most contexts, in particular those involving humans, reflex actions are mediated via the reflex arc; this is not always true in other animals, nor does it apply to casual uses of the term 'reflex'.
- Reaction time