The purpose of a security token in online transactions is to authenticate users and protect sensitive data. These tokens—often generated as strings of characters—serve as a unique identifier that verifies the integrity of a user's session or transaction. By utilizing security tokens, systems can mitigate the risk of fraud and unauthorized access by ensuring only legitimate users can complete transactions. This method enhances the overall security architecture of online platforms, safeguarding both user information and financial transactions in an increasingly digitized and vulnerable operating environment.