A Grimoire is a witch's record of all of her/his spells, rituals, potions, and herbs. It is a spell book filled with information, hexes and spells.
Aldric's grimoire contained all of his spells during the time of ancient Egypt including the immortality spell he created. Aldric gave his grimoire to a powerful witch named Heka, which was pass down through her descendants to Qetsiyah. Silas used the grimoire to turn himself immortal and destroyed the grimoire.
Bonnie's grimoire was passed down from generation to generation and Bonnie uses her ancestor's Grimoire to learn her craft. Jonas Martin had an obsession with Grimoires and collected them. This was important in finding the spell to channel the power of the dead Witches which will possibly lead to the end of Klaus and to find and save his daughter. After Jonas and Luka died, Bonnie packed up their grimoires and presumably kept from in order to kill Klaus. Bonnie wanted to harness the power of all of the dead witches to kill Klaus but if a witch tries to access that much power it would kill them. When Bonnie finally harnessed the power of her ancestors she nearly brought Klaus to the brink of death, but Klaus told Elijah if he rescued him he would show Elijah where he hid their family.
Emily Bennett's GrimoireEdit
[Emily Bennett was a powerful witch in 1864. After she was burned at the stake, her grimoire was taken by the Founding Families, and was later buried in Giuseppe Salvatore's grave. 145 years later, the location of the grimoire was finally discovered by Elena and Stefan, after reading Johnathan Gilbert's journal's. After retrieving the book, Elena and Stefan were forced into giving it to Damon, after he threatened Elena's life. Soon after that, Bonnie and Sheila Bennett were convinced into using the grimoire to try and open the tomb that encased all of the vampires.
During the few episodes when she was still alive, Gloria is seen referring to her grimoire. It's size is found to be bigger than other grimoires shown in the show. Due to her age and experience, Gloria used her powers to scan for a spell without reading anything on it. It may be possible that this grimoire may contain the spell needed to slow down the speed of aging.