一気に is a common adverb meaning 'in one go' or 'all at once.' It emphasizes that an action is done without interruption or that a change happens suddenly and completely. You can use it with many verbs, especially ones involving consumption, movement, or change. For example, you might drink a glass of water in one go, or the temperature might drop suddenly in autumn.

It's a straightforward, everyday expression. You can use it in both speaking and writing, though it's slightly more common in conversation and casual writing. There's no need to adjust the form of the verb; just place 一気に right before the verb or adjective it modifies. Note that it doesn't necessarily imply speed—rather, it's about doing something in a single, continuous stretch.

While it's often used with physical actions like eating or drinking, it can also describe mental or emotional actions, such as reading a book in one sitting, or finishing a task without taking breaks.