What was the weapon Lord Voldemort was after in OotP?

Question

In OotP, when Harry first arrives at the Blacks house, Sirius mentions to him that Lord Voldemort is after a very powerful weapon that he did not have the last time.

'What's he [Voldemort] after apart from followers?' Harry asked swiftly. He thought he saw Sirius and Lupin exchange the most fleeting of looks before Sirius answered.
'Stuff he can only get by stealth.'
When Harry continued to look puzzled, Sirius said, 'Like a weapon. Something he didn't have last time.'

I never quite understood what they meant by this weapon? Did they mean Harry? How would that make sense, how could Harry be used as a weapon by Voldemort?

Answer 1

They mean the Sybill Trelawney's prophecy.

Voldemort tried to kill you when you were a child because of a prophecy made shortly before your birth. He knew the prophecy had been made, though he did not know its full contents. He set out to kill you when you were still a baby, believing he was fulfilling the terms of the prophecy. He discovered, to his cost, that he was mistaken, when the curse intended to kill you backfired. And so, since his return to his body, and particularly since your extraordinary escape from him last year, he has been determined to hear that prophecy in its entirety. This is the weapon he has been seeking so assiduously since his return: the knowledge of how to destroy you.

—Albus Dumbledore to Harry Potter shortly after the Battle of the Department of Mysteries

Answer 2

It was Trelawney's Prophecy about Harry and Voldemort that he was after. The reason the Order thought that he was after a "weapon" was because they weren't really sure what he was after exactly. It seemed like a "weapon" would be the most logical suggestion, considering how dangerous Voldemort is.

Answer 3

I think that the weapon is a piece of writing telling others how to kill Harry Potter. I think it would say things like the only way to kill Harry is with the elders wand... And the only way to get the elders wand is to murder Dumbledore so the weapon must be a piece of writing telling Lord Voldemort how to kill Professor Dumbledore... But what doesn't make sense to me it that Professor Dumbledore is smarter than people think... So he would know that Voldemort is after the piece of writing. So why isn't Dumbledore doing anything about this?... Maybe because he thinks that the writing is on how to kill Harry Potter not him... So he is waiting for Harry to find out what is going on... And when he does find out Dumbledore will tell him everything he already knows...