Strong Customer Authentication (SCA) is a security measure designed to protect online transactions. It requires customers to provide two or more forms of verification before completing a payment. This can include something they know (like a password), something they have (like a smartphone), or something they are (like a fingerprint). SCA is part of the European Union's Payment Services Directive 2 (PSD2) regulations, which aim to reduce fraud and enhance the security of electronic payments. By implementing SCA, businesses can help ensure that only authorized users can access their accounts and make purchases online.