The state or period of not developing, not growing, not going anywhere — the noun that names what happens when something stagnates. Where the verb describes the action, this word names the condition, so it lets you talk about it as a thing: a decade of stagnation, economic stagnation, the stagnation of the industry, to break out of stagnation. That makes it the form you see most in serious writing, especially about economies. 'Economic stagnation' is close to a fixed phrase: growth near zero, few new jobs, wages flat. Journalists also pair it with a length of time ('years of stagnation') because the badness of stagnation is really about duration — one flat year is a pause, ten flat years is stagnation. Beyond economics, it works for careers ('professional stagnation'), for institutions, for art forms, and for personal life ('I needed a change; I could feel the stagnation'). In all of them the same judgement sits underneath: standing still where movement was expected, and going slightly rotten because of it. There is also the literal water meaning — the stagnation of a pond — but that is rare; for water people normally use the adjective stagnant instead. Register is formal-to-neutral, and it is an uncountable noun: say 'a period of stagnation', not 'a stagnation'.