So the extension of the word "dog" is the set of all (past, present and future) dogs in the world: the set includes Fido, Rover, Lassie, Rex, and so on.
所以“狗”这个词的外延是世界上所有（过去、现在和未来）的狗：这个集合包括Fido、Rover、Lassie、Rex等等。