“Synthesis” and “syntheses” are both forms of the same noun, but they differ in their number or quantity.

“Synthesis” is the singular form of the noun, which refers to the act or process of combining different things to create something new. For example: “The synthesis of these two chemicals resulted in a new compound with unique properties.”

“Syntheses” is the plural form of the noun, which refers to multiple acts or processes of combining different things to create something new. For example: “The scientist conducted several syntheses to create a variety of new compounds.”

In summary, “synthesis” is used to describe a single act or process of combining things, while “syntheses” is used to describe multiple acts or processes of combining things.