What is the fix function in math?

Excuse me, could you clarify what you mean by the "fix function" in math? Is this a specific term or concept that I'm not familiar with? Could you provide some context or an example to help me understand what you're referring to? As a practitioner in the field of cryptocurrency and finance, I'm not immediately familiar with a mathematical concept called the "fix function," so I'd appreciate it if you could elaborate further. Thank you!