Staying exactly the same over time — not becoming different, no matter how long you wait. It describes things that hold still while everything around them moves: 'the unchanging routine of village life', 'her unchanging expression', 'the unchanging rules of the game'. It is an adjective, and it usually goes in front of the noun. The feeling it carries depends on the situation. It can be comforting and solid ('his unchanging support for her'), or it can be dull and stuck ('year after year of unchanging work'). Nothing in the word itself tells you which; the sentence around it does. Note it describes a state that lasts, not a single moment — a traffic light that stays red for two minutes is not 'unchanging', but a mountain, a habit, or a law can be.