BGP - definition of External BGP (eBGP)

External BGP or eBGP is implemented between BGP routers that are in different autonomous systems. There is no specialized command to implement an eBGP peering. An eBGP peering is characterized by the fact that the AS argument in the neighbor command is different than the AS of the router command. Note the following BGP configuration:

router bgp 2 bgp log-neighbor-changes neighbor remote-as 3

This BGP router is in AS 2, and it is peering with a neighbor with a remote AS of 3. Thus, this is an eBGP peering.

This is in contrast to iBGP. For a general overview of the difference between the two, take a look at BGP internal vs external.