It's used to indicate the presence of a single atom or molecule of a particular element or compound in a chemical formula.
Examples:
* Monoxide: A compound containing one oxygen atom (e.g., carbon monoxide - CO).
* Monohydrate: A compound containing one water molecule (e.g., copper(II) sulfate monohydrate - CuSO₄·H₂O).
* Monomer: A single molecule that can be joined with other similar molecules to form a polymer.
Let me know if you'd like more examples!