From what I understand, it's what defines the usage of (for example) "bread" for "food". Or "Wheels" for "Car".
Merriam-Webster Online DisctionaryMain Entry:
syn·ec·do·chePronunciation: s&-'nek-d&-(")kE
Function:
nounEtymology: Latin, from Greek
synekdochE, from
syn- +
ekdochE sense, interpretation, from
ekdechesthai to receive, understand, from
ex from +
dechesthai to receive; akin to Greek
dokein to seem good -- more at
-EX-,
DECENT: a figure of speech by which a part is put for the whole (as
fifty sail for
fifty ships), the whole for a part (as
society for high
society), the species for the genus (as
cutthroat for
assassin), the genus for the species (as
a creature for a
man), or the name of the material for the thing made (as
boards for
stage)
-
syn·ec·doch·ic /"si-"nek-'dä-kik/
adjective -
syn·ec·doch·i·cal /-'dä-ki-k&l/
adjective -
syn·ec·doch·i·cal·ly /-ki-k(&-)lE/
adverb