Stoicism (capital S) refers to the ancient school of philosophy founded by Zeno of Citium in Athens, Greece. The philosophy provides a framework of thoughts that gives practicing stoics resiliency during challenging situations. On the other hand, stoicism (small s) as defined by Oxford English Dictionary refers to the endurance of pain or hardships without the display of feeling and without complaint. In other words, the former refers to the school and its teachings while the latter refers to the display of grit and resiliency.
For example, when Chadwick Boseman died many admired his stoicism (small s) because Chadwick never talked openly about his struggles with colon cancer. Ryan Holiday is known for his writings on Stoicism (capital S) and talks about the teachings of the philosophy.
