Naming one thing by another that is connected to it: the part for the whole, the object for its owner, the place for the people, the cause for the effect.
In Romeo and Juliet
A plague o' both your houses!
Mercutio says "both houses" to speak of the Montague and Capulet families, taking the place where they live for the people who occupy it. The word gathers the two groups into a single curse.