Synecdoche (pronounced sin-EK-duh-kee) 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" for cattle, "threads" for clothing, "wheels" for "car", "mouths to feed" for 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" for "home"

• The genus is used for the species
  • "creature" for "person", "personal computer" for "IBM-compatible personal computer"

• 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.