Definitions
Synecdoche is one of the four "master tropes." It uses a part to represent the whole of something, or vice versa, using the whole to represent a part. It can also use a larger group to refer to the smaller group or a smaller group to refer to the larger group.
Examples include calling something by the name of the material it is made of.
Synecdoche is very similar to metonymy but differs in that synecdoche "refers to the whole of a thing by the name of one of its parts".1 Metonymy is also a substitution, but the substitution is suggested by a relationship.
Synecdoche functions to draw the reader's attention through giving common objects or ideas a deeper meaning. It is a type of symbolism that uses one thing to represent something else. Synecdoche focuses the reader's attention on the part, rather than the whole.
Common examples include:
"bread" for money or food
"coke" for all soda and carbonated drinks
"wheels" for cars
"crown" for king or queen
Literary examples:
"Friends, Romans, countrymen, lend me your ears…" In Act 3, Scene 2 of William Shakespeare's play Julius Caesar.2
“I had not intended to love him; the reader knows I had wrought hard to extirpate from my soul the germs of love there detected; and now, at the first renewed view of him, they spontaneously arrived, green and strong! He made me love him without looking at me.” From Jane Eyre by Charlotte Bronte.
Web Resources
“Synecdoche.” Merriam-Webster Dictionary .
Synecdoche in Wikipedia.
“Tropes and Schemes.” Rhetorica: A Rhetoric Primer by Andrew R. Cline Ph.D.