A flask was a container for liquid.
Fenric was trapped in a flask by the Doctor in 3rd century Constantinople and banished to the Shadow Dimensions. (PROSE: The Curse of Fenric)
Bernice Summerfield once found a flask of the Doctor's Napoleon brandy. According to the Doctor, it was really Napoleon's brandy. (PROSE: Sanctuary)
The vapour from a spilled flask of tea helped the Tenth Doctor recover from his recent regeneration. (TV: The Christmas Invasion)