Social norms are the unwritten rules that govern behaviour. They specify what is appropriate, what is expected, what is unacceptable. Unlike laws, they are not enforced by the state. They are enforced by shame, by disapproval, by the simple desire to fit in.
Norm erosion occurs when these rules dissolve without being formally repealed. No one decides that it is now acceptable to cut in line, to litter, to ignore a neighbour in distress. But gradually, incrementally, the expectation that one should not do these things weakens. People observe others violating the norm without consequence. They violate it themselves. The norm dissolves.
The process is self-reinforcing. Each observed violation reduces the perceived cost of violation. The erosion accelerates. What was once unthinkable becomes unusual, then common, then expected. The norm dissolves not because anyone argued against it but because no one defended it.
Some norm erosion is benign. Norms against interracial marriage, against women working outside the home, against homosexuality — these deserved to dissolve. But the mechanism is the same regardless of the content of the norm. Erosion is not a moral process; it is a social one. It dissolves norms indiscriminately, the good with the bad. What replaces them is not necessarily better, only different.