The Earth Defense Network was the name for Earth's planetary defense system in the 23rd century.

In the mid-2270s, V'ger, with knowledge gained from an examination of USS Enterprise's computer, was able to render the network inoperative. This left the planet vulnerable to attack from plasma energy bolts fired by V'ger. Knowledge of the status of this network was broadcast to the USS Enterprise and was seen in a tactical display on the main viewscreen of the bridge. (Star Trek: The Motion Picture)

According to the novelization, had V'ger not gained this knowledge, Kirk was confident that the Earth Defense Network could have held off V'ger long enough to give Earth "bargaining power" to deal with V'ger rationally, despite that V'ger had enough power "to stop the Earth's sun from rotating."
A cut bit of dialogue in the script of Star Trek: First Contact would have established that in 2063, there was were no "Earth defense systems", nor planetary defense systems of any kind.
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