Leaf

 was a maple leaf that caused the meeting of Dave Oswald and Ellie Oswald. On a windy day in 1981 this leaf fell from its tree a flew into Dave Oswald's face as he was walking on a sidewalk. Dave Oswald then stumbled into traffic and was almost hit by a car before being saved by Ellie Ravenwood, his future wife. On a rainy evening, Dave showed Ellie the leaf and said: "...this exact leaf had to grow in that exact way, in that exact place, so that exact wind could tear it from that exact branch and make it fly into this exact face at that exact moment. And if just one of those tiny little things had never happened, I'd never have met you. Which makes this leaf the most important leaf in human history." Ellie would later on take the leaf and put it in the first page of her book 101 Places to See which she passed on to her and Dave's daughter, Clara Oswald, when Clara was nine years old. Later, after saving Clara from having her mind being downloaded by The Great Intelligence, the Eleventh Doctor discovered the book containing the leaf. He asked Clara to explain it and she replied: "That wasn't a leaf, that was page one." (TV: The Bells of Saint John) Clara would later use that leaf to feed Akhaten, a sentient planet feeding off the memories of sentimental things offered to him and on the memories of souls sacrificed to him, overloading him and thus defeating him. She explained by saying: "The most important leaf in human history. It's full of stories, it's full of history. And full of a future that never got lived. Days that should have been but never where. Passed on to me. This leaf isn't just the past, it's a whole future that never happened. There are billions and millions of unlived days for every day we live, an infinity. All the days that never came. And these are all my moments." This was largely a reference to the premature death of Clara's mother and the days of her life she never got to live. As the doctor later explained there is a finite number of events that actually happened, but an infinite amount of possibilities for that which never did happen. This infinite amount of possible memories and moments became too much for Akhaten who imploded from the overload of memories.

The Leaf was destroyed in the process. (TV: The Rings of Akhaten)

behind the scenes
The leaf that was shown in TV: The Bells of Saint John and the one that was shown in TV: The Rings of Akhaten are two different kinds of leaves; the first one, shown in The Bells, is a Maple tree leaf, while the other one is a leaf that can be found more frequently in England than Maple trees. This is probably a production error, though it possible that the two separate leaves could be a narrative clue.