Is a specific configuration of atoms commonly attached to the carbon skeletons of organic molecules and usually involved in chemical reaction.

The answer is "Functional Group"

Explanation: A Functional Group is the arrangement of atoms that characterizes a family of compounds. Commonly is distinguished by different atoms nearly or rounded a carbon atom. Is like the principal actor in a chemical reaction.

For example:

In the picture we can see an Organic Acid named Acetic Acid (commercially known as vinegar). Its functional group (red circle) is -COOH, that means, this arrangement of atoms is the one that characterizes the carboxilyc acids in organic molecules.

take a better picture and ill do it