Stoicism is a school of ancient Greek and Roman philosophy that's enjoying an unlikely twenty-first-century revival in Silicon Valley boardrooms, military barracks, and self-help podcasts. The core idea is austere and useful: the only thing genuinely in your control is your own judgment + response, and serenity comes from radically accepting everything else.
The classical Stoics — Marcus Aurelius, Seneca, Epictetus — were practical philosophers, not abstract ones. They wrote letters, diaries, daily exercises. Memento mori (remember you'll die). Premeditatio malorum (rehearse the bad things). The view from above (zoom out cosmically when small troubles feel large). These practices aren't about emotional suppression — Stoics felt grief, anger, love — but about not being ruled by them.
Modern Stoicism gets criticized for being apolitical (the original Stoics largely accepted Roman slavery), for being too individualistic, for sometimes sliding into bro-grade "be tough" content. The serious version is none of those things — it's a careful framework for keeping your inner life intact while the outer world rearranges itself.