Wikipedia
Thumos (also commonly spelled thymo; ) is a Greek word expressing the concept of " spiritedness" (as in "spirited stallion" or "spirited debate"). The word indicates a physical association with breath or blood and is also used to express the human desire for recognition.
In Homer's works, thumos was used to denote emotions, desire, or an internal urge. Thumos was a permanent possession of living man, to which his thinking and feeling belonged. When a Homeric hero is under emotional stress, he may externalize his thumos and converse with or scold it.1
Plato's Phaedrus and his later work The Republic discuss thumos as one of the three constituent parts of the human psyche. In the Phaedrus, Plato depicts logos as a charioteer driving the two horses eros and thumos (erotic love and spiritedness are to be guided by logos). "In the Republic (Book IV) soul ... becomes divided into nous ("intellect"), thumos ("passion"), and epithumia ("appetite"”). To its appetitive part are ascribed bodily desires; thumos is the emotional element in virtue of which we feel anger, fear, etc.; nous is (or should be) the controlling part which subjugates the appetites with the help of thumos."2 (See Plato's tripartite theory of soul.)
Democritus used " euthymia" (i.e. "good thumos") to refer to a condition in which the soul lives calmly and steadily, being disturbed by no fear, superstition, or other passions. For Democritus euthymia was one of the root aspects the goal of human life.