Webster tells us that emotion is "a psychic and physical reaction (as anger or fear) subjectively experienced as strong feeling and physiologically involved changes that prepare the body for immediate vigorous action". It seems the French word emouvoir is the root, which means "to stir up". You might say that we all get "stirred up" frequently as we go about our daily lives.
My old psychology text teaches that human beings rarely seek goals without fear or joy or happiness or jealousy or anger accompanying that seeking. A quote from the text:
"We live our daily lives in the midst of what may feel like a whole and ever changing symphony of emotional experience."
The physiologist [ different from "psychologist"] teaches that "The Limbic Association Area" is the region of the brain that is concerned primarily with behavior, emotions, and motivation. We all have one in the midbasal regions of the brain. It is the limbic system that provides most of the drives for setting the other of the brain into action.
Another common thread it is... this limbic system.
For other definitions see:
Webster's New Collegiate Dictionary, G. & C. Merrian Co., Springfield, MA, 1981, p.369
For a broad discussion see:
Psychology A Scientific Study of Man, Second Edition, by Fillmore H. Sanford, pp. 253-275.
For those interested in the medical side, see:
Textbook of Medical Physiology, by Guyton, Eighth Edition, p.637.