The Arabic word miftâhî, translated as key, means that which opens or unlocks, is also based on this same root.
In his book, "al-Maqsad al-Asna fi Sharah Asma' Allahu al-Husna" (aka The best means in explaining Allah's Beautiful Names), Imam al-Ghazali translates al-Fattah as "He Who Opens all things".
At times He causes kingdoms to be opened (i.e, conquered) for His prophets, and He takes them out of the hands of His enemies..." Imam al-Ghazzali then quotes a few verses from the Quran to elaborate on this meaning.
One example, cited by Imam al-Ghazzali, is in verse 35:2 which states: "That which Allah Openeth unto mankind of Mercy none can withhold it."
Imam al-Ghazzali says: "At times He lifts the screen from the hearts of His friends and He opens to them the gates to the kingdom of His heaven and the Beauty of His Grandeur."