Adiaphora is one of the more counterintuitive Stoic terms. The Stoics held that only virtue is genuinely good and only vice is genuinely evil. Everything else — and the list is long: health, illness, wealth, poverty, reputation, disgrace, high office, obscurity, life, death — is adiaphoron, indifferent. Not bad. Not good. Indifferent to the goodness of a life.
The doctrine is what keeps Stoic eudaimonia from collapsing into a hostage to fortune — the telos is virtue, and only virtue, so externals cannot make or break the good life.
The doctrine immediately strikes most readers as obviously false. Surely health is better than illness? Surely a long life is better than a short one? The Stoic answer is patient and consistent. Better for what? If you mean better for being a good person — better at being virtuous, courageous, just, wise — then no, health does not do this, and illness does not undo it. Stoics on their deathbeds, Stoics in disgrace, Stoics in exile have demonstrated this. The goodness of the life is not in the externals.
But the Stoics also recognised that, while indifferent to the goodness of the life, externals are not all equally choice-worthy in practice. So the doctrine of preferred indifferents (ta proēgmena adiaphora) and dispreferred indifferents (ta apoproēgmena adiaphora) was developed. Health is a preferred indifferent — worth pursuing, all else equal. Illness is dispreferred — worth avoiding, all else equal. But neither is genuinely good or evil. Both belong to the same category.
The practical force of the doctrine is in two parts.
First, it disarms most of what disturbs us. If you lose money, lose status, lose health — these are dispreferred indifferents, not genuine evils. They are setbacks, not catastrophes. The Stoic does not pretend they are pleasant. He insists that they cannot harm what genuinely matters.
Second, the doctrine is the corrective to pop-Stoic flattening. Common modern paraphrase: don't care about external things. The actual doctrine: care about them appropriately, as preferred or dispreferred indifferents, with a reserve clause, but never as the basis of your wellbeing. The difference between these two is enormous, and the second is what the Stoics actually taught.