Dictionary Definitions from Merriam-Webster's Unabridged Dictionary:

• stoic, the noun
  
1. usually capitalized   : a member of a school of philosophy founded by Zeno of Citium about 300 B.C., extensively systematized by Chrysippus of Soli, and later developed and popularized by Seneca, Epictetus, and Marcus Aurelius
2. : one apparently or professedly indifferent to pleasure or pain : one not easily excited or upset
   
• stoic, the adjective
1. usually capitalized   : of, relating to, or resembling the Stoics or their doctrines
2.  : not affected by passion or feeling;  especially   : manifesting indifference to pleasure or pain
• synonyms see IMPASSIVE
• stoicism, the noun
1. usually capitalized   : the principles or the philosophical system of the Stoics who based an austere ethics on a pantheistic cosmology holding that the world is governed by and is the embodiment of logos, that it is man's duty to conform freely to natural law and his destiny, that virtue is the highest good, and that the wise man should be free from passion equally unperturbed by joy or grief
2.  : the principle or practice of showing indifference to pleasure or pain : repression of feeling : IMPASSIVENESS
   
• impassivity
•  : the quality or state of being impassive : a lack or absence of feeling or expression
  
• impassive, the adjective
1. : devoid of passion, feeling, or receptivity to impression:  a archaic   : unsusceptible to pain, suffering, injury, or harm : INVULNERABLE  b : unsusceptible to physical feeling : INSENSIBLE, INANIMATE
   
2.  : giving no sign of feeling or emotion : EXPRESSIONLESS