Figures of thought differ from figures of speech insofar as figures of thought represent a different way of
conceptualizing a thing, not just phrasing that thing. However, there’s a lot of overlap between figures of speech and figures of thought (for instance, Prince Charles’s phrase, “appalling old waxworks” could well be considered a metaphor, not just
periphrasis). Many people confuse synecdoche and metonymy; while they’re closely related, synecdoche involves treating a literal part of a thing as representative of the thing itself.