When mentioning "external consistency", Brewer references B. Liskov. Practical Uses of Synchronized Clocks in Distributed Systems. ACM Principles of
Distributed Computing (PODC). Montreal, Canada, August 1991.
Liskov, in term, references Gifford D.K. Information Storage in a Decentralized Computer System. Technical Report CSL-81-8, Xerox Corporation,
Here's how Gifford defines external consistency:
External consistency guarantees that a transaction will always receive current information. Using the concepts we have just introduced, we can provide a formal definition of external consistency. The actual time order in which transactions complete defines a unique serial schedule. This serial schedule is called the external schedule. A system is said to provide external consistency if it guarantees that the schedule it will use to process a set of transactions is equivalent to its external schedule.
As @Oleg mentions, this sounds like strict serializability.