User blog comment:Staw-Hat Luffy/Why?/@comment-4605926-20130411111939/@comment-94.68.162.189-20130411132425

Haki (the), noun: a word used by fans of One Piece to describe something that they have no idea what is it. Examples: "What happened in this scene? Haki", "Why the sky seperated in the half? Haki", "How will he see them? Haki"

PS: It is also an ability

Be serious, with Observation Haki you can feel the presence of living things. You cannot identify people, specially when you have never meet 'em. Fujitora cannot identify Luffy, no matter if he is disguised or not.