The short answer is that decadence, for Nietzsche, is being drawn to what is bad for you.
Acting effectively requires self-confidence, great passion to achieve one’s aim, and unity of purpose. To be very successful, and certainly to achieve anything truly great, all of one’s abilities and all aspects of one’s personality must be devoted to achieving one’s aim.
This requires self-mastery, by which Nietzsche means the ability to cultivate one’s drives, desires, and abilities in ways that maximize their contribution to one’s project. Self-mastery is not achieved by conscious deliberation alone; it is an “instinctive” ability to do what is good for you. Faced with a choice, one with self-mastery will identify the best course of action without needing to deliberate.
He guesses what remedies avail against what is harmful; he exploits bad accidents to his advantage; what does not kill him makes him stronger. He collects instinctively from everything he sees, hears, lives through, his sum: he is a principle of selection, he leaves much behind. He is always in his own company, whether he associates with books, human beings, or landscapes: he honors by choosing, by admitting, by trusting. (Ecce Homo, “Why I Am So Wise” §2.)
The decadent, on the other hand, chooses what is bad for him, again in a largely non-deliberative way. A decadent or “corrupt” person instinctively seeks out that which harms him.
I call an animal, a species, an individual corrupt when it loses its instincts, when it chooses, when it prefers things that will harm it. (The Antichrist §6.)
Decadents are disordered, self-deceiving, and impulsive, and they are also dismayed by these defects. Their attempts to remedy them, however, only make them worse. A decadent is drawn to overly simple solutions, which he makes appear plausible by oversimplifying his problems.
Socrates, for example – on Nietzsche’s account – suffered from poor impulse control, ambivalence, self-doubt, and alienation, and he dealt with these disabilities by over-developing his reason and treating his appetites as the enemy. By appointing reason the ultimate master, he made it impossible to achieve true self-mastery.
It is a self-deception on the part of philosophers and moralists if they believe that they are extricating themselves from decadence when they merely wage war against it. Extrication lies beyond their strength: what they choose as a means, as salvation, is itself but another expression of decadence; they change its expression, but they do not get rid of the decadence itself.
Socrates was a misunderstanding; the whole improvement-morality, including the Christian, was a misunderstanding. The most blinding daylight; rationality at any price; life, bright, cold, cautious, conscious, without instinct, in opposition to the instincts – all this too was mere disease, another disease, and by no means a return to “virtue,” to “health,” to “happiness.” To have to fight the instincts – that is the formula of decadence: as long as life is ascending, happiness equals instinct. (Twilight of the Idols, “The Problem of Socrates” §2.)
Signs of decadence include guilt: watch out for people who constantly find themselves doing things they don’t approve of. Take care around those who suffer from suffering: who not only can’t get what they want, but want not to want what they want.
Now, has the culture become decadent?
Nietzsche sometimes talked about decadent cultures or peoples in the more or less ordinary sense of the term: they have lost the discipline required to achieve their values, or have lost interest in the values themselves. Laws are not enforced, standards are lowered, difficult things are not attempted.
But Nietzsche also contrasts “healthy” and “sick” cultures. Healthy cultures acknowledge the value of competition, risk-taking, and even aggression and violence. These are ineradicable features of human nature, they go along with a robust love of life, and outlets must be provided for them.
A sick culture, on the other hand, is devoted to sapping people’s vitality and rendering them weak and infirm. This applied, Nietzsche thought, to the Victorian culture of his own day: people were made to feel guilty for the desire to stand out, and either inhibited themselves or tried to stand out by virtue-signaling and denouncing the selfishness of others.
Is our culture healthy, sick, or decadent? It strikes me as a chaotic bundle of all three. There are plenty of competitive strivers, and no lack of aggression. There are many who enjoy naming and shaming. And there’s sufficient lawlessness and incompetence to suggest that some have lost the courage of their convictions. None of these practices fits well with the others. Since chaos and disunity are the essence of decadence, perhaps that’s the best overall characterization.
Decadence in the ordinary sense of decline, Nietzsche thought, isn’t necessarily all bad. When a culture loses enthusiasm for its ideals, the drives, dispositions, and desires repressed by the declining regime of value begin to express themselves. New values are discovered or created. A period of decadence can be a breeding ground for “free spirits.”