Fixed at inception, and the reference point for moneyness.
Traders name strikes by delta, not by price. A "25-delta put" is a strike whose delta is -0.25, which is roughly a 25% chance of finishing in the money. This is preferred because it stays meaningful as the underlying moves and across different assets, whereas a fixed price does not.
Quoting in delta terms also makes strikes comparable across expiries, since a 25-delta strike is much further from spot at one year than at one week.