To gulp is to loudly and quickly swallow food, drink, or air. You might gulp nervously before you stand up to make a speech.
When you gulp down your breakfast so you can run and catch your bus, or gulp air upon rising to the surface of a swimming pool, you can say you take a gulp — or many gulps. The word dates from the 1300s, but its origin is somewhat uncertain. Most experts link it to the imitative Dutch gulpen, "to gush, guzzle, or swallow."