Social engineering is the act of manipulating people into performing actions or divulging confidential information. While similar to a confidence trick or simple fraud, the term typically applies to trickery or deception for the purpose of information gathering, fraud, or computer system access; in most cases the attacker never comes face-to-face with the victim.
But technically, I know it is used in many fields for many purposes. Basically, controling and directing human behavior. We do this in our day to day lives really, but it is used systematically in government/politics, the military, and especially businesses and marketing.