We've provided the key terms used throughout the documentation for easy reference.

API KeyThe X-Api-Key which is provided in the API headers is used to uniquely identify you as a merchant in our system. This key is highly sensitive and should be kept safe.
Bearer TokenA Bearer Token is an obfuscated string (not intended to have any meaning to clients using it). Some servers will issue tokens that are a short string of hexadecimal characters, while others may use structured tokens such as JSON Web tokens.
Payment TokenThe payment token is a way to uniquely and securely identify a payment instrument which has been stored in the vault. Payment tokens are automatically issued during the tokenization process and will be used when processing payments, setting up payment agreements etc in place of sensitive card data.
Payment InstrumentA payment instrument when referred to throughout our guides and references is used for any payment method which has been tokenized.

For example Credit Cards, Gift Cards and PayPal are all considered Payment Instruments.
Step Up TokenA step up token is a temporary unique and secure reference to a captured CVV. As per PCI DSS we are not allowed to store CVV details in our vault so where you as a merchant require a CVV to be captured for credit card payments we will return a step up token which should be used when making the payment.

Please note that step up tokens are single use and once used to attempt to process a payment will be deleted and a new Step Up Token will be required.