Short answers, pulled from the story.
Emotions are physical and mental states brought on by neurophysiological changes, associated with thoughts, feelings, behavioral responses, and a degree of pleasure or displeasure. There is no scientific consensus on a single definition. From a mechanistic view, emotion has been defined as a positive or negative experience associated with a particular pattern of physiological activity.
The word emotion dates back to 1579, adapted from the French emotion, which comes from the Old French emouvoir, meaning to stir up. The modern concept was coined in the early 1800s by Thomas Brown, and the modern idea of emotion emerged in English around the 1830s.
Paul Ekman's facial-expression research examined six basic emotions: anger, disgust, fear, happiness, sadness, and surprise. For more than 40 years he has argued that emotions are discrete, measurable, and physiologically distinct, and that certain emotions are universally recognized even in preliterate cultures.
The James-Lange theory holds that emotions are feelings that result from physiological changes rather than causing them. William James argued in his 1884 article that the perception of bodily changes, as they occur, is the emotion, so a person feels afraid because they tremble. The Danish psychologist Carl Lange proposed a similar theory at around the same time.
Neurobiology locates emotion in the limbic system of the mammalian brain, including the hypothalamus, cingulate cortex, and hippocampi, identified through work by Paul Broca in 1878, James Papez in 1937, and Paul D. MacLean in 1952. The amygdala coordinates behavioral responses to threat, while love involves networks including the ventral tegmental area, thalamus, substantia nigra, putamen, caudate nucleus, and anterior cingulate cortex.
Lisa Feldman Barrett's theory of constructed emotion argues that emotions are socially constructed rather than triggered. She says emotions emerge from a combination of the physical properties of your body, a flexible brain that wires itself to its environment, and your culture and upbringing.
Bharata Muni enunciated the nine rasas, or emotions, in the Natyasastra, an ancient Sanskrit text written between 200 BC and 200 AD. They include Shringara for romance and love, Hasyam for laughter, Raudram for fury, Karunyam for compassion, Bibhatsam for disgust, Bhayanakam for horror, Veeram for pride and heroism, and Adbhutam for surprise and wonder.