The term Upstream is used in 3scale documentation but not defined in 3scale Glossary.

Following are some usages in APIcast standard policies

• Upstream - "upstream URL defined in the Private Base URL with a different URL"
• Upstream Mutual TLS - "connections between APIcast and upstream APIs" .
• Upstream Connection configures NGINX directives . "allows you to change the default values of the following directives, for each API, depending on how you have configured the API backend server in your 3scale installation" 

Define and add  "upstream"  to the Glossary