AP is calling Israel's attack on Lebanon an invasion. What does that mean and why does it matter?

The Associated Press is now calling Israel’s military actions in southern Lebanon an invasion. Israel has moved thousands of troops across the border into Lebanon, and Israeli forces and Iran-backed Hezbollah militants have been fighting on the ground for at least three weeks. Israeli officials, including the defense minister, said they want to take control of the entire area south of the Litani River -- some 20 miles (about 30 kilometers) north of the border – and that displaced Lebanese residents will not be allowed back until Israel decides that its northern border is safe.