The technology primarily used for secure online transactions is SSL/TLS (Secure Sockets Layer/Transport Layer Security). These protocols encrypt the data transmitted between a user's browser and the website server, ensuring that sensitive information, such as credit card details or personal data, remains confidential and protected from malicious actors. Implementing SSL/TLS is crucial for building user trust and is often indicated by a padlock icon in the browser’s address bar. Without this security layer, online transactions could be vulnerable to man-in-the-middle attacks and data breaches.