Definition of "line of defense" [line of defense]

• Any organization whose responsibility it is to defend against something (noun)
• Defensive structure consisting of a barrier that can be employed for defense against attack (noun)

WordNet. Princeton University. 2010.

• "The next line of defense appeared to be the North Anna River, twenty-three miles north of Richmond."
• "This was to provide one more line of defense against things that could kill a crew, like not being able to close the Payload Bay Doors PLBD."
• "He displaced the unreliable riverain tribes north to Dongola, as the first line of defense against invasion from Egypt."