Bear with me. I’ll come back to this later. From the Oxford English Dictionary:

synecdoche
A figure by which a more comprehensive term is used for a less comprehensive or vice vers’; as whole for part or part for whole, genus for species or species for genus, etc. Formerly sometimes used loosely or vaguely, and not infrequently misexplained.

The definition from Wikipedia is pretty good:

Synecdoche is a kind of metonymy in which:

• A part of something is used for the whole,
• The whole is used for a part,
• The species is used for the genus,
• The genus is used for the species, or
• The stuff of which something is made is used for the thing.

Some common examples of synecdoche:

• A part of something is used for the whole

• “hands” to refer to workers, “head” to refer to cattle, “threads” to refer to clothing, “wheels” for “car”, “mouths to feed” to refer to hungry people

The whole is used for a part

• “the police” for a handful of officers, the “smiling year” for “spring”, “the Pentagon” for the top generals in the Pentagon building

The species is used for the genus

• “cutthroat” for “assassin”, “kleenex” for “facial tissue”, “castle” is used for “home”,

The genus is used for the species

• “creature” for “man”, “personal computer” for “IBM-compatible personal computer”, “colored” for a particular color

The stuff of which something is made is used for the thing

• “hickory” for “baseball bat”, “copper” for “penny”, “boards” for “stage”, “ivories” for “piano keys”, “plastic” for “credit card”

Synecdoche, as well as other forms of metonymy, is one of the most common ways to characterize a fictional character. Frequently, someone will be consistently described by a single body part or feature, such as the eyes, which comes to represent their person.

Also, sonnets and other forms of (erotic) love poetry frequently use synecdoches to characterize the beloved in terms of individual body parts rather than a whole, coherent self. This practice is especially common in the Petrarchan sonnet, where the idealised beloved is often described part by part, from head to toe.

As I said, I’ll probably be coming back to this concept.