Stoicism is a branch of philosophy founded by Zeno of Citium on the Stoa in Athens in the early 3rd centure BCE. It is a philosophy of personal ethics informed by a system of logic and views on the natural world. (1)
If you had to describe Stoicism in one sentence:
"A Stoic believes they don't control the world around them, only how they respond - and that they must always respond with courage, temperance, wisdom, and justice."