Which term describes the way an organism responds to stimuli?

Reflex Action is the term that describes the way an organism responds to stimuli. During reflex actions receptors are stimulated, they send signals to the central nervous system, where the brain co-ordinates the response. But sometimes a very quick response is needed, one that does not need the involvement of the brain. Reflex actions are rapid and happen without us thinking. For example, you would pull your hand away from a hot flame without thinking about it.

The term that describes the way an organism responds to stimuli is called behaviour.

Behaviour patterns may be simple or complex. Behaviour is explained through proximate causation and ultimate causation. In proximate causation, it is how behaviour works or ensues, internal state delivers physiologic basis, it is a study that determines physiologic changes like hormone levels. One example is a male songbird singing during mating season, this is because of the level of sex hormone-testosterone which binds to certain receptors in the brain which activates production of song. Ultimate causation on the other hand, explains why behaviour evolved through the years, it studies the behaviour in which it influences survival or reproduction. One example is a female bird during mating season who are mostly attracted by singing male birds and male songbirds have more offspring compared to those who don't influenced that shape behaviour.