Albus Dumbledore's knife was a silver knife owned by Albus Dumbledore. In June 1997, Dumbledore used the knife to cut his hand in order to get some blood as a tribute or payment to open the sealed archway entrance to the seaside cave where he and Harry Potter retrieved the fake Horcrux locket. Dumbledore offered the knife to Harry to use again for the doorway's blood tribute when they fled the cave, although since Harry had already scraped his arm on a rock, it was not necessary.
The Noble Collection sells a replica of the knife. The prop used in the movie appears to have been based on an French knife design, the Opinel.