Oak-matured mead is a high-quality alcoholic beverage enjoyed in the wizarding world. Mead itself is an alcoholic drink created by fermenting honey with water. The “oak-matured” descriptor indicates that it has been aged in oak barrels, a process that typically imparts complex flavours and a higher value, similar to high-end Muggle spirits. The specific bottle mentioned in the story is not described in detail, other than being a gift-worthy item that Horace Slughorn was saving for a special occasion. Slughorn considered it a “good pick-me-up” and suitable for a celebratory toast.
In its unadulterated state, oak-matured mead is simply a fine beverage. However, the bottle acquired by Horace Slughorn was a vehicle for a very potent and fast-acting poison.
While mead is a common drink available at establishments like the Three Broomsticks Inn, the story of this particular bottle is an integral part of Draco Malfoy's attempts to assassinate Albus Dumbledore during Harry Potter's sixth year.
The poisoning via the oak-matured mead is a crucial event in Harry Potter and the Half-Blood Prince.