When the air stands still and there is no perceptible movement, it's called a dead calm. This term can be used to describe various scenarios, including the lack of wind, waves or tides. However, there are several other synonyms available that can be used in its place, such as stagnant, motionless, hushed, still, lifeless, and tranquil. These words help to create a vivid picture of what's transpiring, painting a picture of utter stillness and calmness. The use of synonyms not only adds variety to your writing but can also help to enhance the imagery, making it more immersive for the reader.