喻底 用英语怎么说？

metaphor or simile；analogy；figure of speech；allegory
这些是比喻的意思

本体 - Topic
喻体 - Vehicle
喻底 - Ground

"The conventional referent of the unit is the Vehicle.
The actual unconventional referent is the Topic.
The similarities and/or analogies involved are the Grounds."（Goatly, 1998, p8)

Ref:
Goatly, A., 1998. The Language of Metaphors, London: Routledge.