The legal team of the International Committee of the Red Cross publishes a definition of occupation on the ICRC website. It reads as follows:
Article 42 of the 1907 Hague Regulations . . . states that a territory is considered occupied when it is actually placed under the authority of the hostile army. The occupation extends only to the territory where such authority has been established and can be exercised.
As the recent war between Israel and Hamas makes self-evident, the Gaza Strip does not meet this definition. If it did, Israel would not have had to respond to rocket fire and tunneling into Israel with an invasion of the Gaza Strip. The Gaza Strip has been under the authority of Hamas, the Palestinian Islamist group, since 2007. There has been no Israeli military presence there since 2005.
Furthermore, according to UN Security Council Resolution 242, passed in 1967, the West Bank and other territories captured by Israel in the 1967 war are classified as administered territory whose ownership is disputed and whose status will only be finalized in a comprehensive peace agreement.