The term "vast" typically refers to something that is extensive in size, extent, or scope. It can describe a wide range of contexts, such as:
Physical space: A vast area of land, ocean, or sky.
Volume: A vast quantity of something, like water or resources.
Time: A vast period, such as a vast history or a vast expanse of time.
Concepts or ideas: A vast array of possibilities or a vast intellect.
In a sentence, "vast" might be used like this:
"The ocean is a vast expanse of water, stretching for thousands of miles."
If you have a specific context or example in mind where you want to use "vast," please provide more details, and I can tailor the explanation to that context.