What is a reaction to stimuli or environment called?

The pattern is stimulus produces a response. The reaction to a stimulus is called a response, sometimes even an unconscious or involuntary response.

Examples:

I feel cold.

I pull the blanket over me.

A man is sleeping; he has his hand under his head and the hand begins to lose circulation so it is tingling.

Without awakening, the man changes position.

You touch a hot stove by accident.

Immediately (without thinking to do it), you pull your hand away.

A boy takes a big gulp of milk, but the milk has soured.

Immediately (without thinking), the boy experiences a gag reflex and spits the soured milk out of his mouth.

Social conditioning and response to a stimulus:

While walking from the bus stop to your house, your neighbor yells, "Hi" and waves.

You respond, "Hi" and wave back.

Your teacher sneezes.

Most people would say, "Bless you", even if the person responding is not religious.

A CEO of a corporation agreed to meet the 10th grade class and their teacher.

As the teacher approaches, the CEO extends his right hand.

The teacher responds with his hand... they shake hands.

The CEO goes down the row of students to shake their hands. Many students are not used to being treated like adults, so they awkwardly shake hands with the CEO. By the time a dozen students have done it, the rest of the students know what is expected, so they easily shake hands with the man. (a conditioned response to a stimulus)