Imagine this scenario. You have bought an IP or DIA circuit from someone that is going to provide your network with bandwidth. Typically this company will make the connection, IP wise, over a /30 or even a /29 of IP space. I have called this the “glue address” for many years. This is the IP address that binds (the glue reference) you to the other provider’s network. They can route you IP blocks over that glue address or you can establish BGP across it, but it is the static address which binds the two networks together.
Some network folks call this a peering address. This isn’t wrong but can infer you are doing BGP peering across the address. You aren’t always doing BGP across the glue address.